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Do pleco’s and bristlenose’s need wood? Pseudoxylovory in Loricariidae

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This is a commonly mentioned topic when it comes to pleco’s, it is a bit of a telephone game over time even since many scientific studies have disproven it. There are three aspects to cover here can they digest it, do they need it for digestion and does wood have other benefits?

Figure 1: Panaque schaeferi at Maidenhead Aquatics Ascot

The Background

Loricariids are a group of around 1,080 currently accepted and described species according to Eschmeyer’s Catalog of Fishes, they are endemic to Central and South America. While common names within this group are unreliable it generally includes pleco’s, whiptail catfishes, oto’s, bristlenoses etc. Here in this article I am focusing on a subfamily known as Hypostominae, this includes some of the pleco’s (the most popular ones) and bristlenose pleco’s (Ancistrus, which is nested well within).

Figure 2: Pterygoplicthys joselimaianus at Maidenhead Aquatics Ascot.

Some Loricariid genera were observed to gouge on wood using their jaws, Loricariids do not suck their food unlike the name suckermouth catfish suggests. They use a pair of oral jaws and basically rasp at it (Adriaens et al., 2009), which is then further processed by some pharyngeal jaws (a second pair of jaws) who often contain small teeth (Bentley et al., 2026). These paraphyletic genera displayed spoon shaped teeth (Fig. 1; spoon shaped) not found in other groups (Fig. 2).These teeth are unique within Loricariidae for being able to gouge into the wood and often mineralised for strength (Schaefer & Stewart, 2009).

Genera associated with wood eating:

  • Panaque in the Hemiancistrus medians group close to genera such as Baryancistrus and Parancistrus (Lujan et al., 2015; Roxo et al., 2019).
  • Panaqolus was previously in Panaque but identified to be in the Peckoltia group also with genera such as Scobinancistrus and Hypancistrus (Lujan et al., 2015; Roxo et al., 2019).
  • Hypostomus in the Hypostomus cochliodon group, so not far from Hypostomus plecostomus.
  • Pseudoqolus, also in the Peckoltia group (Lujan et al., 2015; Roxo et al., 2019).
  • Potentially Lasiancistrus heteracanthicus, sister genus to Ancistrus (Lujan et al., 2015; Roxo et al., 2019).

These genera have uniquely been identified to also have wood in their stomach (Lujan, German & Winemiller, 2011; German, 2009), aquarists will notice their faeces will also reflect this and, any wood will usually slowly be broken down faster then if there was not these fishes. This doesn’t tell the full picture as many animals including animals consume things that we do not digest, use for digestion or any other purpose.

Questioning their ability to digest wood

The issue is xylovory, the ability to eat wood is quite rare and unseen in fishes so any occurrence would be fascinating to study. In the initial German (2009) experiment neither Panaque nigrolineatus (Royal pleco) hypothesised to be a wood eater and Pterygoplichthys disjunctivus (common pleco) which was used as a detritivore retained any wood in their gut and the fishes actually lost weight throughout German (2009) experiment where the fishes were only provided with wood. In fact the wood passed through their digestive tract in only 4 hours so not nearly enough time to digest wood (German, 2009), in contrast I identified around 2-4 days for Ancistrus on an algivore diet but in Bentley et al. (2026) Planiloricaria took around 4 days for food and even substrate to pass through the gut.

Panaque cochliodon, blue eye royal pleco at Maidenhead Aquatics Ascot.

Anatomical issues

Regardless of differences in jaw and tooth shape neither Panaque or Pterygoplichthys actually differed in intestinal anatomy (German, 2009), showing they are unable to digest the wood using an anatomical method. Teeth and jaw shape only allow for so much processing and, even gut anatomy only allows for so much too. In a way the anatomy should just speed up processing, which is why those animals which swallow prey whole have that food in their stomach and then intestines a long time. Wood is particularly difficult to digest due to the lignin and cellulose although this is present in many plants it’s what creates the structure to these plant structures so you’d expect a longer gut should the fish be able to digest wood and in fact it was also shorter for Panaque (German, 2009), I hypothesise this is due to Panaque consuming more microbes, fungi and in some habitats algae (Lujan et al., 2011) opposed to Pterygoplichthys who possibly consumes more algal matter but is also much more of a generalist (Stolbunov et al., 2021) as also consuming plant matter and some invertebrates, perhaps if both feed on algae’s it could infer a difference in algae’s as the vast group differ in structure. Regardless other then jaw anatomy, the wood ‘eating’ species don’t differ.

Are they actually processing any of this wood?

While many studies have identified wood in the guts of these fishes it doesn’t entirely say what is being digested, many organisms consume different items for a variety of reasons and not all are there for a purpose of which I will discuss later. This is where isotopes are useful as these can indicate what food is actually being processed and assimilated. Lujan et al. (2011) used isotopes to identify that of the wood consumed they were not digesting it, instead they were actually digesting the microbes and even the fungi within the wood. This study was the first to not just use Panaque and Panaqolus but additionally used a member of the Hypostomus cochliodon group.

This inability to process wood had additionally been shown in Pterygoplichthys which has never had wood in the gut within Pterygoplichthys disjunctivus (German and Miles, 2010).

Enzymes

German and Bittong (2009) identified that neither Panaque nigrolineatus (Royal pleco) and Pterygoplichthys disjunctivus had nearly enough of the enzymes (cellulase and xylanase) to digest wood.

Panaqolus albivermis, Flash/Emperor pleco. Likely female

Gut microbes

This aspect has been debated a little but gut bacteria can significantly influence what we can digest but also what we eat influences the gut bacteria. Neither Panaque nor Pterygoplichthys contained enough gut bacteria that would allow for the digestion of wood when cultured (Nelson et al., 1999); in Panaque nigrolineatus when genetic analyses of gut micromes were assessed (McDonald et al., 2019) or; in a larger study using the DNA and RNA of Panaqolus albomaculatus, Panaqolus gnomus, Panaqolus nocturnus and Panaque bathyphilus (McCauley et al., 2020). When comparing to the wood, those microbes alive in the gut used in digestion also differed but it did leave the opportunity that they are digesting the microbes within natural wood.

Niche Partitioning

All of this evidence (Lujan et al., 2011; McCauley et al., 2020; German, 2009; German and Bittong, 2009; McDonald et al., 2019) proves that there are no xylovorous Loricariids, pseudoxylovory could be a good term but in fact they are detritivores. As earlier the majority of Loricariids are algivores or detritivores (Lujan et al., 2012) which might disappoint a lot of people but this actually says something quite interesting. Loricariids are a massive group of fishes, well over 1,000 species described according to Eschmeyer’s Catalog of Fishes, many are found in the same area. As you might well know, species can’t often co-exist if competing. While these pseudoxylovores (‘wood-eaters’) are not competing due to the use of wood they are actually not competing due to likely where these fishes can find their food, these fishes can find their food in wood, where none of the others actually can. This partitioning of dietary niche is maybe more common in Loricariidae but we don’t really know, there are many algivore’s in the same locality and it’s likely each is eating different algae’s, there is a diversity of jaw and tooth shapes afterall so maybe some are consuming say diatoms and others filamentous algae.

Panaque are actually found in habitats where there isn’t any wood (https://amazonas.dk/index.php/articles/brasilien-rio-xingu), so they are certainly capable of generalisation. Within the aquarium Panaqolus and Hypostomus cochliodon group have proven to be hardy, good growers and long lived and while it is difficult to assume, they might not always be kept around wood. While we like to think of species as specialists, while they are to some extent many also are capable of generalisation, such as why many who are really great at consuming one food item take guinea pigs are great with grass are more then capable at eating a wide range of different plant matter, even if it might not be as good nutritionally to depend on this. This is a massive topic discussed in evolutionary biology for a variety of species.

The reason pleco’s don’t eat wood is why we have so many species available to us. It’s why they are so interesting for evolutionary biology.

Hypostomus basilisko, rusty pleco. Member of the Hypostomus cochliodon group

Of course this topic actually got wider, like the telephone game people changed their ideas. While no other Loricariid other then these three(ish) genera consume wood, it got extended to a pick and mix of most Loricariids.

So they will eat the microbes within aquarium wood?

This is a common topic brought up, while it is logical we have to remember that we tend to use a very niche range of woods in the aquarium hobby, we also want them to last as long as possible. Microbes within wood are generally there to break down this wood and ideally we, as aquarists want to avoid this so we actively choose woods that last a long time in the aquarium and this means that there will be minimal microbial growth. They obviously just don’t decay quickly enough if lasting years to provide many microbes. We also tend to sterilise the wood beforehand and choose very hard woods, for a variety of reasons aquarium woods are unlikely to have these microbes. McCauley et al. (2020) used similar methods and actually struggled to find microbes involved in wood decay in the gut but also they might not have been able to identify microbes being digested but allowed them to identify any microbes used to digest wood.
Personally, I collect fresh and dried woods of a variety of ‘safe’ trees such as palms, beech, birch, oak and fruit trees, all actually break down quickly sometimes within a year, even before that they become softer and easy to break. So it is clear that most aquarium woods are of not much use, I also personally keep Panaqolus (P. albivermis and P. aff. maccus) and while wood consumption differs they really don’t consume that much. I also find particularly Panaque maybe due to their size prone to stunting when really not fed enough which is common as many do depend on the wood, when I’ve fed them daily or twice daily they do grow much more rapidly.

So they need it for digestion?

This has been mentioned a lot but actually has no backing in science or actually really any reliable origin. I assume it relates to if they cannot digest it, why is it in the gut and many will think about gastroliths, stones within the gut that are used to aid in digestion. While an interesting thought, wood is only found in these few genera, given they are digesting the ‘norm’ compared to the majority of Loricariids (Lujan et al., 2012), why are the others not consuming wood even where wood is present? Many lack the teeth to do so as well, so haven’t evolved the mechanism of consuming even a small amount of wood. There are Loricariids who consume tougher food items from seeds, insects and even molluscs like snails, but many of these have completely different jaw shapes likely aimed at extracting the soft tissues from these items or large pharyngeal jaws. It’s not always well known Loricariids have two pairs of jaws, this means they can actually chew almost twice (Bentley et al., 2026). With the pharyngeal jaws the use of some material to digest makes little sense but also why wood consumption is not so common throughout the group.

Panaque schaeferi, royal pleco at Maidenhead Aquatics Ascot.

Consuming items for no purpose

Why is the wood in their gut if it has no purpose? This is likely due to what they are digesting and the niche partitioning, to consume the bacteria, fungi and other microbes they are digesting (Lujan et al., 2011), just to consume these food items with their mouth it is likely to pass through the gut.

One item that doesn’t seem to get discussed is substrate, it is found throughout Loricariid guts (Lujan et al., 2012), so sand is an interesting concept for Loricariids although they definitely have no issue without substrate. So it’s likely again they are consuming a food item because they are foraging around it so it just passes through the gut. This is also observed in Labridae specifically parrotfish which seem to eat sponges but in actual fact they are digesting the microbes within these sponges (Crossman et al., 2005) as identified by Professor Donovan German (https://german.bio.uci.edu/PFK_response.html).

Panaqolus aff. maccus, one of the clonw pleco’s. Likely female.

Just because it is fed on doesn’t mean it is consumed, other Loricariids.

The issue has become wider though, although originally it was restricted to those pseudoxylophagous species although after the last few years it has spread to a paraphyletic pick and mix of Loricariids. The majority of these species lack the teeth or jaws to consume wood, quite a lot also aren’t found around wood in the wild such as the gold nugget pleco (Baryancistrus xanthellus) who is an extreme algivore almost exclusively found around rocks (Py-Daniel et al., 2011).

This idea is often backed by the idea if the fish are rasping on something or feed on it maybe they are eating it. But this is somewhat flawed, these fishes are evolved to rasp on a surface (Adriaens et al., 2009), it doesn’t mean they eat that surface or find a benefit on rasping from that individual type of surface. Finding some of your food in one place doesn’t mean you depend on this place particularly when algae and detritus grows on a variety of surfaces.

Is there a place for wood?

Ofcourse, it can be a hiding space for many species, it’s also complex in shape so allows for many more different hiding spaces. For those that are considered pseudoxylophagous, while they don’t digest the wood and it’s not used for digestion there is still a behavioural benefit to provide it as it acts as enrichment, it would also be interesting to provide caves made of wood for these species.

So why is this topic an issue?

Initially it seems harmless, wood could provide enrichment benefits so why not provide it? But the harm actually comes from two aspects 1) depending on it as a food source and 2) assuming the lack of it as a cause of death. The first aspect is maybe clearest on Panaque who seem so easily stunted for a generalist, it shouldn’t take 15-20 years for them to reach adult size, particularly when compared to their relatives but with such poor food provision it seems to be the excuse used. The second aspect is more of a feedback issue, when fishes die the assumption the lack of wood is a cause detracts from the actual cause and can mean that that issue is not solved. Loricariids aren’t the super hardy fishes they are often labelled as, they are diverse and complex so we still need to identify issues with water quality, disease, food provision, nutrition etc. blaming the presence of wood solves nothing.

References:

Adriaens, D., Geerinckx, T., Vlassenbroeck, J., Van Hoorebeke, L., & Herrel, A. (2009). Extensive jaw mobility in suckermouth armored catfishes (Loricariidae): a morphological and kinematic analysis of substrate scraping mode of feeding. Physiological and Biochemical Zoology, 82(1), 51-62.

Bentley, R. F., Collins, R. A., & Genner, M. J. (2026). Evolutionary innovation of functional fused molariform pharyngeal jaws in suckermouth catfishes. Evolutionary Journal of the Linnean Society, kzag009.

Crossman, D. J., Choat, J. H., & Clements, K. D. (2005). Nutritional ecology of nominally herbivorous fishes on coral reefs. Marine Ecology Progress Series, 296, 129-142.

German, D. P. (2009). Inside the guts of wood-eating catfishes: can they digest wood?. Journal of Comparative Physiology B, 179(8), 1011-1023.

German, D. P., & Bittong, R. A. (2009). Digestive enzyme activities and gastrointestinal fermentation in wood-eating catfishes. Journal of Comparative Physiology B, 179(8), 1025-1042.

German, D. P., & Miles, R. D. (2010). Stable carbon and nitrogen incorporation in blood and fin tissue of the catfish Pterygoplichthys disjunctivus (Siluriformes, Loricariidae). Environmental Biology of Fishes, 89(2), 117-133.

Lujan, N. K., Armbruster, J. W., Lovejoy, N. R., & López-Fernández, H. (2015). Multilocus molecular phylogeny of the suckermouth armored catfishes (Siluriformes: Loricariidae) with a focus on subfamily Hypostominae. Molecular phylogenetics and evolution, 82, 269-288.

Lujan, N. K., German, D. P., & Winemiller, K. O. (2011). Do wood‐grazing fishes partition their niche?: morphological and isotopic evidence for trophic segregation in Neotropical Loricariidae. Functional Ecology, 25(6), 1327-1338.

McCauley, M., German, D. P., Lujan, N. K., & Jackson, C. R. (2020). Gut microbiomes of sympatric Amazonian wood‐eating catfishes (Loricariidae) reflect host identity and little role in wood digestion. Ecology and Evolution, 10(14), 7117-7128.

Nelson, J. A., Wubah, D. A., Whitmer, M. E., Johnson, E. A., & Stewart, D. J. (1999). Wood‐eating catfishes of the genus Panaque: gut microflora and cellulolytic enzyme activities. Journal of fish biology, 54(5), 1069-1082.

Py-Daniel, L. R., Zuanon, J., & Oliveira, R. R. D. (2011). Two new ornamental loricariid catfishes of Baryancistrus from rio Xingu drainage (Siluriformes: Hypostominae). Neotropical Ichthyology, 9(2), 241-252.

Roxo, F. F., Ochoa, L. E., Sabaj, M. H., Lujan, N. K., Covain, R., Silva, G. S., … & Oliveira, C. (2019). Phylogenomic reappraisal of the Neotropical catfish family Loricariidae (Teleostei: Siluriformes) using ultraconserved elements. Molecular phylogenetics and evolution, 135, 148-165.

Schaefer, S. A., & Stewart, D. J. (1993). Systematics of the Panaque dentex species group (Siluriformes: Loricariidae), wood-eating armored catfishes from tropical South America. Ichthyological Exploration of Freshwaters, 4(4), 309-342.

Stolbunov, I. A., Gusakov, V. A., Dien, T. D., & Thanh, N. T. H. (2021). Food spectrum, trophic and length-weight characteristics of nonindigenous suckermouth armored catfishes Pterygoplichthys spp.(Loricariidae) in Vietnam. Inland Water Biology, 14(5), 597-605.

Panaque schaeferi, royal pleco at Maidenhead Aquatics Ascot.

Fish Diseases: Hole in the Head

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Hole in the Head (HITH) is a disease but like swim bladder disorders more of a symptom rather then a pathogen, it’s quite distinctive but not always the easiest to handle. Most commonly found it seems in cichlids particularly discus but I have personally identified the disease in goldfish. It is related to lateral line erosion.

Hole in the head disease in a domestic discus (Symphysodon), some pits show further progression of the necrosis. Image obtained from aquabase, http://aquabase.uwu.ac.lk/fishDisease?id=9

Identification

Can be characterised by:

  • Holes/pits on the head or lateral line.
  • Appears first white growths which last usually a few days from these pits.
  • Necrosis around the the pits.
  • Often in older fishes particularly cichlids (Amesberger‐Freitag et al., 2019; Morrison et al., 2007; Paull and Matthews, 2001).
The white precursor growths to hole in the head. Sourced from Fishlore.

It is definitely worth noting those white growths, some have confused it with being caused by fungi, bacteria or other protozoans. Some unreliable resources have thought it’s a totally new disease (Canal Neuromast Inflammation Syndrome, which isn’t a real disease) but if you actually observe it is the early stages of hole in the head.

Causes

The lateral and neural pits of a fish where HITH and lateral line disease occurs/originates. Sourced from White Shark Projects.

The disease involves erosion and necrosis of areas where neural pits are present, this causes tissue damage and necrosis of the surrounding tissues giving originally a pitted appearance but can spread further.

The actual causes are debatable and there are many ways to handle it:

  • Hexamita and related pathogens such as Spironucleus (Paull and Matthews, 2001), pathogens have been isolated from these lesions.
  • Stray charge, not sure if this has any scientific backing but it is worth just crossing out as it’s quite easy to do so.
  • Activated carbon (Amesberger‐Freitag et al., 2019), largely associated with any could have powders rather then the sponges. While it seems shown in a marine sturgeon fish (Acanthurus bahianus) which generally are associated with a different pathogenic cause and the mode of action isn’t well known there is some correlation (Stamper et al., 2011). I wonder if it’s associated with actually attaching to the pits or causing some abrasion but I don’t know how it would work.
  • Nutrition (Amesberger‐Freitag et al., 2019; Paull and Matthews, 2001)
  • Age and other stressors (Amesberger‐Freitag et al., 2019; Morrison et al., 2007)

Treatment

My personal opinion is to actually cross off as many causes as possible, nutrition and water quality should be assessed, is the diet providing everything the fish needs. While I do think some diets are good for rapid growth for long term health they might not be as useful. Stray charges can be grounded and it has actually solved the issue for me in the past, but any faulty electrics are worth checking for. Activated carbon is largely not needed long term in an aquarium unless removing tannins or medication, it is useful for filtration of rainwater though.

When it comes to actually treating Hexamita many have found Octozin previously sold by Waterlife but now NT labs effective containing dimetridazole. It is directed to add into the water but there is discussion online to be used orally with food particularly on persistent infections, as it is now in a liquid it should be quite easy for the fish to take up and easy to avoid underdosing. It is always best and advised to treat as recommended on the bottle. In the US or countries where legal to be obtained without a prescription metronidazole be used (Whaley & Francis-Floyd, 1991), this is also an antibiotic so will target a very broad group of organisms and shouldn’t be used without considerable thought.

Prognosis

At the end of the day this disease can be tricky to treat, it is often not fatal. It can lead to more permanent scars and the fish if it is not severe can heal. It can also be lethal if left. So it is definitely worth treating but it doesn’t progress as rapidly as say white spot/ich (Ichthyophthirius multifiliis) so not quite as lethal.

References

Amesberger‐Freitag, A., Tichy, A., El‐Matbouli, M., & Lewisch, E. (2019). Hole‐in‐the‐head disease in discus fish, Symphysodon (Heckel, 1840): Is it a consequence of a dietary Ca/P imbalance?. Journal of Fish Diseases, 42(8), 1133-1142.

Morrison, C. M., O’Neil, D., & Wright Jr, J. R. (2007). Histopathology of “hole-in-the-head” disease in the Nile tilapia, Oreochromis niloticus. Aquaculture, 273(4), 427-433.

Paull, G. C., & Matthews, R. A. (2001). Spironucleus vortens, a possible cause of hole-in-the-head disease in cichlids. Diseases of Aquatic Organisms, 45(3), 197-202.

Stamper, M. A., Kittell, M. M., Patel, E. E., & Corwin, A. L. (2011). Effects of full‐stream carbon filtration on the development of head and lateral line erosion syndrome (HLLES) in ocean surgeon. Journal of Aquatic Animal Health, 23(3), 111-116.

Whaley, J., & Francis-Floyd, R. A Comparison of Metronidazole Treatments for Hexamitiasis in Angelfish IAAAM 1991.

Blackwater Aquariums, what is blackwater?

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Blackwater is one of the most popular biotypes of current years although it has been one of the most commonly known. Biotypes being more loose then biotope, just fishes which originate from similar habitats but could be from different localities. Although blackwater is diverse so maybe more then a biotype. There have been many misconceptions about blackwater for various reasons and this is possibly due to the popularity but also as entire regions become generalised as blackwater.

This article is not really going to be about biotopes, more then often most blackwater setups are not biotopes.

What is blackwater?

Blackwater is most simply defined as an aquatic habitat stained by tannins but are low in suspended sediment (Meyer, 1990). Tannins are also known as tannoids and tannic acids. These tannic acids result in a higher turbidity. Most often these habitats are drained from regions with sandy soils that do not retain any of the organic matter so it is leached into the water. These rivers are often low gradient (Meyer, 1990) but actual size of the river varies given the Rio Negro and .

Figure 1: Water parameters of different water bodies (Flotemersch et al., 2024)

The water chemistry of blackwater habitats can be varied in conductivity and pH and therefore hardness (Fig 1; Flotemersch et al., 2024). Although they are typically lower in conductivity and pH this is not always the case.

Frequently cited will be Alfred Russell Wallace’s publication on his travels within South America. When inspecting the book there is no mention of water parameters and this is not surprising as the ability to test water parameters was already limited in the 1850’s, particularly for a naturalist of a modest background but also in the field would entirely be impractical. In fact, the book is very vague as to the hydrology (Wallace, 1853). So while he was analysing species within the Rio Negro, Brazil he wasn’t the first to identify these stained waters, in fact many have given the number of towns and rivers labelled as blackwater and later noted for being of this physiology.

Figure 2: South American stream hydrology/physiology (Albert et al., 2025).

Blackwater habitats as previously mentioned are more typical with specific river hydrological physiologies and distributions are often based on these. Figure 2 displays the distribution of blackwater habitats across Northern South American freshwater ecosystems, defining them mostly at lower flow habitats.

What are tannins?

Tannins are one of the many compounds produced by plants to reduce predation by herbivores, often bitter tasting compounds. These compounds are what stains your black, green and red teas which actually disproves the idea that tannins can only exist in soft, low pH waters but it is easier to obtain in those softer waters as any tea drinker might know.

These compounds are a bit more complex as anti-herbivory mechanisms using a bitter taste. Tannins are antinutrients, limiting protein uptake (Becker and Makker, 1999; Hassan et al., 2020; Wang et al., 2024; Kokou and Fountoulak, 2018) although could also damage other physiological processes (Liu et al., 2025; Omnes et al., 2017). While a more well known benefit is that tannins are also antimicrobial (Wagner et al., 2012; Balta & Balta, 2024), like the antinutrient effect it will be concentration dependent. While there are studies on both looking at these effects of tannins, we don’t know the concentrations of these compounds in our aquariums and it makes it impossible to assume if there is an antimicrobial or antinutritional effect when present within our aquariums. This unpredictability is increased by the fact that other compounds influence the turbidity/staining so we can’t assume that darker stains mean increased antimicrobial/antinutritional effects over a tank with reduced staining as the concentrations of tannins to change the stain differ.

The effect of hardness

What you might have noticed is that there is no mention of hardness or conductivity in my definitions. There is a reason why, there are hard water and brackish blackwater habitats but these are less common but this doesn’t mean they don’t exist and many definitions allow for this (Flotemersch et al., 2024; Hackney et al., 2022; Bonotto et al., 2025) . Blackwater as a term is useful as a definition for tannin rich waters as distinctive habitats as these tannins effect productivity just by light penetration but it’d require a shift in the other terms whitewater and clearwater into much broader categories.

Apistogramma cacatuoides, dwarf cockatoo cichlid female.

Tannins are actually reasonably common in daily life, these are the compounds which stain teas, beers and wines with their distinctive range of red, yellow and brown tones. If you are a tea drinker you might know that you can make a stained tea whether you have hard or soft water, just if you have hard water often you need a higher concentration. The tannins are still released as the plant breaks down, this is no different from in nature. Maybe the misconception that blackwater is only found in softer waters is also the perception that all freshwater habitats are full of leaf litter and botanicals which is a photographical bias regarding popular freshwater media. But there is an incredible diversity of freshwater habitats and situations, so while there will be white/clear waters with leaf litter and submerged plant matter not all do. What probably switches a river towards being blackwater further is the amount of external humic acids draining from outside the river into it, this is confirmed with Meyers (1990) definition. Most of these rivers at the end of the day seem to flow through heavily forested areas so it is likely a strong influence from the external and internal plant matter. But as earlier stated don’t confuse a highly turbid (low clarity) river with being blackwater, there are many rivers with higher sediment loads or other reasons clarity can be low such as just being lower down the stream order with a lot of silt and maybe tidal which kicks up a lot of that silt.

A clearwater aquarium using leaf litter for Pangio spp.

Differences in plant species and structures

Different species of plants contain different concentrations of tannins, this is kind of well known as you might have your preference of species but also noticed some produce a deeper stain more easily then others. What is probably lesser known is different plant structures produce different levels of tannins and this can vary seasonally, this is why plant saplings can be more toxic then their leaves at lower doses. This isn’t a guide on how to use botanicals but I recommend this article on species to use: https://www.practicalfishkeeping.co.uk/features/free-tank-decor/ Just be careful with articles as many do not cover the positives and negatives of plant compounds and many of the health benefits could be overstated in the concentrations we are using.

Red cover domestic discus, Symphysodon sp. x.

Personally I do not collect botanicals for their stains and I do not use leaves as I keep largely clear/whitewater fishes personally. Alder cones (from Alnus spp.) are one of the best examples to release tannins that you can collect whereas for leaf litter oak leaves (Quercus spp.) are often distinctive. I have also found palm leaves from Trachycarpus fortunei seem to release over a long time a weaker tannin content. But I do largely collect fresh and dried woods from safe deciduous trees such as oak, beech, birch and sycamores.

Plant compounds

Tannins seem the most well known of the plant compounds to the aquarist, these are a form of polyphenol but plants produce a whole diversity of different compounds with a wide range of different properties. It is worth investigating the various compounds of different plants before adding them to the aquarium in larger quantities, dried or fresh because drying doesn’t largely remove them unless you have a way of draining them. This is a massive topic and while many probably wont kill a fish rapidly and depend on how they are introduced to the fish it is worth considering. One example is calcium oxalates found in many aroids such as Anubias and pothos, these are toxins which require being ingested and particular volumes so while they cause damage it is only a concern in larger volumes but we don’t know how much as it’s largely unknown how much many different aroid species contain.

The blackwater adaptation

Fishes that inhabit blackwater habitats likely have adaptations to cope with the physiological stressors of tannins and other plant compounds so they might be able to benefit from those positives such as antimicrobial activity. Perhaps it could be from a higher protein requirement but then due to antimicrobial properties they might not have evolved an immune system that can cope with higher microbial loads. It is likely species dependent. Recognised adaptations is to the lower oxygen saturations within blackwater habitats which is likely a barrier for other species (Small et al., 2014) but blackwater species could be less adaptable to temperatures higher then their normal range (Braz‐Mota et al., 2025).

Conclusion

Blackwater habitats and freshwaters are unusual habitats classified by their ‘tea’ coloured staining caused by compounds known as tannins. Tannins are complicated and there are negatives as well as positives. At the end of the day if you have fishes from blackwater habitats there is likely a benefit.

References:

Albert, J. S., Abrahão, V., Akin, D. R., Allen, J. G., Ândrade, M., Arce, M., … & Reis, R. E. (2025). An ecological trait matrix of Neotropical freshwater fishes. Scientific data, 12(1), 1127.

Balta, Z. D., & Balta, F. (2024) Determination of Antimicrobial Activity and MIC Value of Tannic Acid Against Four Different Fish Pathogens. Journal of Anatolian Environmental and Animal Sciences, 9(4), 582-589.

Becker, K., & Makkar, H. P. S. (1999). Effects of dietary tannic acid and quebracho tannin on growth performance and metabolic rates of common carp (Cyprinus carpio L.). Aquaculture, 175(3-4), 327-335.

Bogotá-Gregory, J. D., Lima, F. C., Correa, S. B., Silva-Oliveira, C., Jenkins, D. G., Ribeiro, F. R., … & Crampton, W. G. (2020). Biogeochemical water type influences community composition, species richness, and biomass in megadiverse Amazonian fish assemblages. Scientific Reports, 10(1), 15349.

Bonotto, D. M., Lunardi, M., & Goonetilleke, A. (2025). Hydrochemistry of Blackwaters in a Shoreline Zone of São Paulo State, Brazil. Journal of Marine Science and Engineering, 13(8), 1575.

Braz‐Mota, S., Duarte, R. M., & Val, A. L. (2025). Contrasting thermal and hypoxic responses of species from blackwater and whitewater rivers. Journal of Fish Biology.

Hackney, C. T., Adams, S. M., & Martin, W. H. (Eds.). (1992). Biodiversity of the southeastern United States: aquatic communities (pp. xiii+-779).

Hassan, Z. M., Manyelo, T. G., Selaledi, L., & Mabelebele, M. (2020). The effects of tannins in monogastric animals with special reference to alternative feed ingredients. Molecules, 25(20), 4680.

Kokou, F., & Fountoulaki, E. (2018). Aquaculture waste production associated with antinutrient presence in common fish feed plant ingredients. Aquaculture, 495, 295-310.

Liu, J., Zhang, X., Lu, Q., Zhang, H., Lin, L., & Li, Q. (2025). Tannic acid reduced the growth performance, antioxidant, and immune functions of the Nile tilapia (Oreochromis niloticus). Aquaculture, 596, 741872.

Meyer, J. L. (1990). A blackwater perspective on riverine ecosystems. BioScience, 40(9), 643-651.

Flotemersch, J. E., Blocksom, K. A., Herlihy, A. T., Kaufmann, P. R., Mitchell, R. M., & Peck, D. V. (2024). Distribution and characteristics of blackwater rivers and streams of the contiguous United States. Water resources research, 60(2), e2023WR035529.

Omnes, M. H., Le Goasduff, J., Le Delliou, H., Le Bayon, N., Quazuguel, P., & Robin, J. H. (2017). Effects of dietary tannin on growth, feed utilization and digestibility, and carcass composition in juvenile European seabass (Dicentrarchus labrax L.). Aquaculture Reports, 6, 21-27.#

Ríos-Villamizar, E. A., Adeney, J. M., Piedade, M. T. F., & Junk, W. J. (2020). New insights on the classification of major Amazonian river water types. Sustainable Water Resources Management, 6(5), 83.

Small, K., Kopf, R. K., Watts, R. J., & Howitt, J. (2014). Hypoxia, blackwater and fish kills: experimental lethal oxygen thresholds in juvenile predatory lowland river fishes. PLoS One, 9(4), e94524.

Wallace, A. R. (1889). A narrative of travels on the Amazon and Rio Negro: with an account of the native tribes, and observations on the climate, geology, and natural history of the Amazon valley (No. 8). Ward, Lock.

Wagner, E. J., Oplinger, R. W., & Bartley, M. (2012). Evaluation of tannic acid for disinfection of rainbow trout eggs. North American Journal of Aquaculture, 74(1), 80-83.

Wang, Y., Wu, J., Li, L., Yao, Y., Chen, C., Hong, Y., … & Liu, W. (2024). Effects of Tannic Acid Supplementation of a High‐Carbohydrate Diet on the Growth, Serum Biochemical Parameters, Antioxidant Capacity, Digestive Enzyme Activity, and Liver and Intestinal Health of Largemouth Bass, Micropterus salmoides. Aquaculture Nutrition, 2024(1), 6682798.

Social Media, where to go for advice?

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Social media is tricky when looking for fishkeeping advice, there are a lot of sources and all with their pro’s and con’s. It’s even more tricky for a beginner but I assume most reading this website are not beginners, even the most seasoned aquarist can fall into echo chambers and feel lost with knowing where to go. I hope this article can offer some help.

Some might know that I do not just have this website, I have a Youtube channel, run multiple Facebook groups and can be active on Discord. Previously I was on a few forums and sometimes will frequent the Planet Catfish forum. Social media is hectic and confusing, it can be full of drama but where isn’t that has people?

At the end of the day many fishkeepers don’t frequent a lot of social media, Youtube particularly is very focused onto those who can make videos which takes skill and time.

So what social media is there?

Throughout this article I wont include channels/groups that encourage poor welfare, out of date or inaccurate/unreliable ideas. This is not the extensive list as no doubt I’ll have missed some.

Youtube/Instagram

I would say both of these platforms are quite similar. I largely am not keen on either for learning, they are entertainment platforms. Viewer count is not about knowledge (or experience) but about entertainment value. Youtube is tricky as there is no real argument for or against, on it’s own largely it’s self focused so ideas cannot really be contested or debated. There is little ability to cite sources and it is not frequently done.

Both are so much of a bottleneck that the most knowledgeable fishkeepers inevitably will not be the most popular. The most seasoned aquarists seem to frequent other sources.

Saying all of that there are great channels, those that show different biotopes or discuss the science.

My favorite channels:

  • Professor Leandro Souza, (https://www.youtube.com/@LeandroSousa_IctioXingu). A great channel for anything connected to the Rio Xingu, it is largely in Portuguese but subtitles exist. Not just can you see the habitats of the various fishes but the fishes themselves.
  • Chris Englezou/CE Fish Essentials, (https://www.youtube.com/@cefishessentials). Great for biotopes and some real thought provoking discussion.
  • Cam and John, The Fish Room (https://www.youtube.com/@TheFishRoom). Their weekly livestream/interview is one of the best podcasts around. They have interviewed some of the top names of fishkeeping from Ian Fuller to Project Piaba (and some Rift Valley cichlid people) and really care for advancing knowledge. If fishes aren’t entirely your thing many episodes involve interviewing brands and aquascapers.
  • Meenkaran, (https://www.youtube.com/@flare1979/). If you want to see anything from biotopes to husbandry and science of South East Asian fishes, no further needed to go.

Knowledgeable fishkeepers:

I put this as separate as they are kind of a different format and knowledge to be gained.

  • Alyssa Bentley, (https://www.youtube.com/@mangala666). One of the best Loricariid breeders, she is currently building a fishroom after many years of fishkeeping.
  • Dylan/DJA Aquatics, (https://www.youtube.com/@DJAAquatics). A Loricariid breeder with a lot of knowledge and passion for this group of fishes and Corydoradinae.
  • Graeme/Aquarium Adventures, (https://www.youtube.com/@AquariumAdventures). A great channel for fishkeeping in general, a channel that emphasizes the fact we are always learning as fishkeepers. If you want to try a different aquarium sealant other then silicone, a great channel to watch.
  • Paul/BigFishLad, (https://www.youtube.com/@bigfishlad4847). Good for anyone interested in keeping those larger Central/South American cichlids, what is required and a channel that shows high welfare or thoughtful care of these fishes.
  • StephenP, (https://www.youtube.com/@StephenP2003). Stephen has a real interest for plants and learning more about them. Not just the husbandry but also the science behind them.
  • Tropical Fish Hub, (https://www.youtube.com/@tropicalfishhub). A great channel largely focused on South American fishes. Looks more into their husbandry but has a lot of high welfare and thoughtful setups.
  • Bills Cichlid Room, (https://www.youtube.com/@BillsCichlidRoom). This channel is great for seeing a realistic fishroom with a particular focus on cichlids. Bill obviously has a lot of knowledge about a variety of freshwater fishes but particularly cichlids.
  • Amiidae, (https://www.youtube.com/@amiidae). This channel shows a lot of footage of rarer species from a diversity of locations.

Discord Servers:

Don’t forget we have our own discord server! The link to join: https://discord.gg/CzFRvEQkrT

Might Scottish Law Ban Aquariums and Other Exotics?

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While it has been a hot topic in the exotic pet community for a while this topic is starting to gain traction in the aquarium hobby. The Scottish Government was provided with a report by the SSPCA, Born Free Foundation and One Kind titled “Don’t Pet Me”. This report sets out a case for the restriction and banning of a number of pets although definitions are unclear they largely use the term wild animals.

When analysing a document like this it’s important to understand who wrote it, unlike scientific papers this is not peer reviewed and biases do not have to be declared. Born Free Foundation is already actively against exotic pets (https://www.bornfree.org.uk/wild-animals-pets/), this is also the case for OneKind (https://www.onekind.org/listing/category/dont-pet-me) and the Scottish SSPCA is largely an animal welfare organisation but has done a lot of really good work. The others have done misleading campaigns in the past but I wont get into them as the focus here is this one. None of the contributors seem to be stakeholders in the industry nor scientists studying the relevant taxa.

The Reports Methodology

The first thing to set out is what they mean by a wild animal. This report fails to provide a definition for a wild animal, or as they say an exotic pet, I shall be using these terms interchangeable. This lack of clarity is problematic as the definition and understanding of the term varies. The Cambridge dictionary states a wild animal as “An animal that lives in natural conditions and is not cared for by humans”, which is a broad brush but ignores any captive animals as they are obviously being cared for. UK law doesn’t seem to have a definition beyond a list of those species covered by the Dangerous Wild Animals Act 1976 which require a license. Exotic pets in an interim report with the Scottish government identified the term is misleading and inappropriate with varying definitions (https://www.gov.scot/publications/scottish-animal-welfare-commission-interim-report-exotic-pets-scotland/pages/5/). A debate in the UK parliament identified exotic pets as rare or unusual https://commonslibrary.parliament.uk/research-briefings/cdp-2015-0124/, not a helpful definition. A 2022 final report mentions rabbits and small rodents as exotic pets although doesn’t focus on these excluding dwarf hamsters (https://www.gov.scot/publications/final-report-exotic-pet-working-group-scottish-animal-welfare-commission/pages/4/). When it comes to vets it’s generally considered anything that is neither livestock, horses, a dog or cat is considered exotic. With such a broad definition this makes this 2025 report handed to the Scottish Government concerning but also open.

While literature is present on the topic of a variety of exotic pets, this report also fails to reference much of this peer reviewed scientific literature. Much of the literature provided shows a clear conformation bias rather then an argument, it only portrays one point of view. There is no discussion of the benefits and many of the citations reference trends in other countries opposed to focus on practices in the UK. Some of the sources are irrelevant e.g. the captive husbandry of slow loris (Nekaris et al., 2015) in page 31 and used it to compare to crested gecko’s, could they not find a single source that inferred that the squeaking behaviour was an example of stress in this gecko species? Just because viewers might find a behaviour cute it doesn’t mean they want to buy the animal. Ramsay et al. (2007) is used to discuss red eared sliders, a species already banned in the UK. Realistically given the very limited citations there isn’t so much I can say about them.

They claim to send a mystery shopper to a range of stores as mentioned in page 8, while it sounds ideal how do we know what stores were included and the quality of those stores? Was it just the bad eggs? Could they not say which stores they visited?

Figure 2: Some definitions provided by the Don’t Pet me report, 2025.

There are some issues with the terminology used (Fig 2), maybe it is too simplistic but they could have cited a source with a detailed definition. The one that is most odd to me is morphs, not all morphs are ‘sought after’, this phrase maybe makes it seem like there is a trend or market. A morph really is just a variant. The term morph is very specific to the reptile hobby and seems more like targeting as within other exotic pet hobbies we might use terms such as breeds or variants.

The Reports Results

Some of these results would benefit from reliable citations such as “For example, royal (ball)
pythons (Python regius) are frequently wild-caught”, given the number of morphs in the UK which would not be wild caught as these are captive strains this is likely misleading. Additionally yes there will be royal pythons wild caught but are these reaching the UK? The paper that the report they mention mostly doesn’t infer this is relevant (Green et al., 2020) as the source of royals into the UK isn’t mentioned. This could be solved by asking wholesalers.

The report criticises the use of RUB’s (Really Useful Boxes), the reptile hobby already criticises this and it’s a self policing hobby that is regulating, yet there is little mention of this. This self policing trait within exotic pet communities while sometimes internally criticised is an important part of developing and improving welfare based on science and evidence within a hobby (Muka, 2022), it is not rare either. Maybe this is evidence of how the worst parts of our communities let us down, and could now be our downfall. My community here, on Youtube and Discord is not alone in providing up to date husbandry that also helps self police to the hobby. This has been somewhat recognised within the report for the hedgehog hobby (Page 12) but it makes no effort to recognise how widespread this is. There is frequent criticism of snake enclosure sizes, maybe supporting and promoting those who are encouraging evidence based husbandry would be better then making it seem like the whole hobby is doing this.

Some of the images used (e.g. Page 13) are not from the UK or represent the UK trade, many parrots in the UK will be captive bred. This is emotive imagery but not realistic.

Figure 3: Criticism of transportation by the Don’t Pet me report, 2025.

Fishes are often transported as we are well aware in bags sometimes polyboxes but these are safe for the fish in movement and there has yet to be a good alternative. The mention of bedding and ventilation for shipping fishes is just ridiculous, what bedding would they suggest? And ventilation? It’s just as bad as PETA suggesting we use tranquilisers to transport fishes. Any decor does risk breaking the bag but also if it shifts stressing out or damaging the fish.

Figure 4: Criticism of shop labelling by the Don’t Pet me report, 2025.

Some of the criticism maybe is a little odd (Figure 4), labelling a species as easy to keep or beginner friendly doesn’t mean the store isn’t recognising there is species specific requirements, it’s inferring that the husbandry is not as difficult as other species. While I disagree often with the species chosen it is not for this reason, often these species might have requirements that can be missed by someone after an ‘easy’ pet but no hobbyist usually wants their animals listed as easy.

Figure 5: Don’t Pet me report, 2025.

While a problematic scenario is provided (Figure 5) when provided with a challenging situation such as a child wanting a pet the second solution is not bad. Customers can be challenging and this second solution likely would allow the child to obtain some education from the animal, avoid an unwanted pet and hopefully get the parents involved. But without a full script much of this secret shopping experience is biased and unreliable. The report fails to recognise that sales setups are aimed to be short term and therefore often smaller then a long term setup and these are not recommended for long term care.

Issues with Data

There are frequent issues with the data provided which could be misleading but also infer to me a lack of knowledge about animal biology and husbandry.

Figure 6: Number of individual species sold from the Don’t Pet me report, 2025.

Data often needs to be properly dissected and understood or it can be misread. Figure 6 identifies cherry shrimp (Neocardinia davidi), Guppies (Poecilia reticulata, do they mean P. wingei hybrids as well?) and cichlids as the most populous species sold. There are loads of errors with this statistic. Firstly cherry shrimp and guppies are very frequent spawners and small organisms as hobbyists we know this, so it makes sense so many would be for sale. But given their size they take up much less space and instead particularly for guppies a focus would be better aimed as only buying males given females are often already pregnant. I don’t see how these two are a problem entirely being so high, they are easy going aquatics. Cichlids though is an issue, you first have the bias that this is one of the largest families of fishes with incredible diversity. So it is understandable why there would be so many. I do have an issue with data like this where there is not equitable groups provided. The fishes shown here represent likely multiple species so would represent more individuals, is that bad?

Figure 7: Top five advertised per category and number of animals in the Don’t Pet me report, 2025.

Figure 7 is very problematic to me as none of these groupings really have definitions. My criticism of the term guppy without a scientific name is the same from the previous comment. Unknown species could infer the seller doesn’t know the species, or the writers of the report have no idea, if they can’t identify the species why are they writing a report on husbandry? Is it to make the sellers look bad if it infers that the seller doesn’t know? Discus are cichlids and ‘Plecostomus’ are catfishes. The discus category is concerning as it’s pretty simple, wilds aren’t common in the UK so we are dealing with domestics which are a hybrid, it’s not even unknown they are a mixture of mostly Symphysodon tarzoo and S. aequifasciatus (Ng et al., 2021).

Plecostomus as someone who studies Loricariids is the most problematic category. Plecostomus is no longer a valid genus, Hypostomus plecostomus is barely if at all in the trade. Plecostomus as a common name is a pick and mix name, so how broad do they mean? Could they be including the Chinese algae eater, Gyrinocheilus aymonieri, which is a very frequently rehomed fish. Are they including Ancistrus as some have done? What about Loricariinae? 92 (Figure 7) would be a lot even just for Pterygoplichthys which has never been in Plecostomus. Loricariidae is popular but the term Plecostomus means very little without a definition. Many people breed Loricariids and according to the IUCN what is identified, there are still many threatened in the wild. I’m not being pedantic though, this is not a consistent term.

Exotic pet demographics

As someone who has been diagnosed with dyslexia and SpLD since I was 8 this section infuriated me. There was an obvious targeting of neurodiverse people.

Figure 8: Mentions of Neurodiversity in the Don’t Pet me report, 2025.

An unneeded targeting of neurodiversity was included (Figure 8), why this was relevant I have no idea. While they recognise precise knowledge it seems to infer there is something problematic with neurodiverse people and animals, that maybe we are less capable. Neurodiversity includes conditions from autism, dyslexia, ADHD and similar, it’s a broad term but we all think differently, this means we can problem solve. These conditions do not infer with our ability to keep animals with high welfare and in fact it might mean we can identify new solutions to husbandry problems.

They continue to target disabilities.

Figure 8b. Criticism of the disabled community in the Don’t Pet me report, 2025.

There is no need to explain how animals help with mental health, we all know that. Although I argue fishes are stress causing haha, I must say Zach, my little (6kg! but not overweight) void cat has been a blessing to me (even if he ensures he has breakfast at 2-4am no matter what). Exotics can differ a lot from traditional pets so it makes sense many provide solutions to disabled people, it could be from cleanliness and issues with fur or just fascination. This statement (Figure 8b) infers that disabled people, that includes the neurodiverse community so those with dyslexia, ADHD and autism can’t cope with their animals because we are disabled. Most of us our disability has nothing to do with animal husbandry! While I have short term memory issues, issues with understanding language it has nothing to do with how I can keep my animals. Even when I’ve broken bones I’ve still managed to do water changes, in a way my dyslexia has allowed me to problem solve around not being able to use a broken arm/wrist. It ignores how disabled people can identify new solutions due to differences in thinking. I don’t know what disabled people are frequently saying they can’t cope as that’s definitely a minority. Neurodiversity is common in fishkeeping and no one has told it’s causing them to struggle. I’ve worked in the trade and I know exactly how much I can care for and I know I can keep a lot more then I do now (academia means moving a lot).

Another concern of the report which highlights a bias is they recognise on page 21 that many keepers work with animals, qualifications vary from none to PhD. Yet in the next paragraph they state “Most people who own wild animals appear to be either irresponsible, or well-intentioned but lacking the necessary knowledge and equipment.”. This is clearly stating they think that those who work with the animals husbandry are not as knowledgeable, as who? I assume them? Do they have anything to back this up? And given Born Free has frequently campaigned against zoos it infers that they are inferring there are no husbandry experts. It infers that zookeepers and store staff, academics etc. don’t know as much as they do.

Figure 9: Don’t Pet me report, 2025.

I don’t know why Figure 9 was included, is this inferring that people who show their animals are of a low socio-economic status? And what do they mean by that? How are they judging? You can’t judge from a video or photo someone’s socio-economic status.

Figure 10: Don’t Pet me report, 2025.

No one is debating the external influences with the hobby (Figure 10), the hobby actively tries to fight it yet little mention or promotion of responsible communities.

Frequently throughout statements are made but not cited or even a lack of values provided. They state their opinions but make it unclear that they are opinions. They do not communicate with husbandry or scientific experts in a variety of groups. Frequently statements are made about the hobby that the hobby is trying to solve.

Page 29 fails to recognise the lack of availability of exotic vets or specialism of vets particularly for diverse groups. It also fails to recognise responsible communities and individuals recommend vets frequently where this is the required intervention and not a husbandry issue.

They state people blame “Flimsy enclosures were often blamed but human error was also a cause.”, both of these can easily be the cause as there are weak enclosures and an owner might not be aware. This is obviously more common with small mammals but with fishes a lid is always promoted for a variety of reasons.

On page 29 they identify the use of live feeding vertebrates, the legislation on this is tricky but it would be great if they could provide clarity as to if their sources were in the US or other countries where this is common. I have yet to hear of this in the UK.

I guess this is a positive for fishkeeping!

They do mention issues with handling on page 31, as someone whose worked with a range of exotic animals but also a fan of choice based handling. It can be useful for animals to get used to handling as sometimes care might be required, this is less common for non-mammalian exotics who lack nails or fur that needs trimming. They mention axolotls being handled which I have yet to see being common practice……. although aquatic species being handled for some such as Loricariids it allows for a quick clear assessment of the fish and as facultative air breathers it causes little harm, most people are only taking photos when being moved between setups so will be out of the water anyway. Seeing defensive as ‘cute’ is not entirely common but for some animals if handling is required confidence is needed as waiting, going in slow will likely result in an accident.

Premature death in the trade is mentioned but there are no real statistics on this and this should be recognised.

For demographics my personal concern has been the age groups between around 16-18 which will be moving for university or perhaps renting, not that they can’t keep pets but it does effect choices. Having animals younger effects where you can live given rental restrictions but also animals being banned from university halls. This has not been identified in the report yet within aquariums and reptiles there is definitely a demographic gap where there are less people between around 18-30 although I would be curious how the upper end might change.

The reports recommendations

A Positive List

The most concerning part of the report is the promotion of a positive list, this is a list of species that people would be allowed to keep. We have no idea what would be included here and the report provides little clarity other then more common exotics such as rabbits, guinea pigs and hamsters maybe wont be included? This is regardless of those three also facing similar difficulties but also have active communities promoting welfare.

This positive list could mean many things, it could mean that you could keep what you have till it passes. It might mean you cannot breed, trade or sell the animals. It could mean euthanasia. It could mean no imports. Most of these would make legal captive populations extinct.

Illegal populations will always exist but these will go underground, animals will have no access to veterinary care. While exotic vets are rare there will be even less access to them for the animals that exist legally as demand decreases, many of these animals are long lived so catfishes, parrots and reptiles would be without.

The benefits of the aquarium hobby are well known, both to people but also to the species encountered within it (Evers et al., 2019). As a hobby we do need to promote and further these benefits, we need to reduce the pressure on a small number of species but be responsible with what we keep. The hobby is a source of knowledge both from hobbyists (Marchio, 2018) who are not professionals but also from academics and husbandry experts, it has been identified previously vets will defer to hobbyists for knowledge (Walster et al., 2015).

Encounters, awareness and exposure to a diversity species will decrease. Zoos contain a very limited number of exotics for many groups such as freshwater fishes. A lot of interest in many freshwater groups which can lead to scientific research or conservation (ex-situ and in-situ) can originate from this exposure. There are many aspects of the aquarium hobby involved in ex-situ conservation and captive breeding so a positive list would remove this potentially causing an extinction of some species. Captive breeding fishes is promoted within the hobby and often not for money (Pountney, 2023).

The aquarium hobby provides an industry for locals who fish for the species we value as hobbyists. None more obvious then the cardinal tetra and discus of South America such as around Manaus. The aquarium hobby provides a job for locals, without it to support their families they would have to go into other jobs such as deforestation or mining.

The aquarium hobby, it’s exposure and the structure itself has allowed for the identification of new species, while not always classified we have systems such as the L number and CW number system. These can support and aid scientific research.

What can you do to protect your hobby?

While this is a Scottish report and aimed at restricting animals within Scotland there is no harm in making your concerns known throughout the UK. Contact your local MP with your concerns. Within Scotland be aware of who you are voting for.

OATA has great sources as the only representative for UK fishkeeping who is able to fight this report: https://ornamentalfish.org/

Amazing figure provided for OATA of the impact of the aquarium hobby in the UK from 2024-2025.

The report itself: https://dontpetme.org/ https://drive.google.com/file/d/1nvje1bJ0x0ESLKbRuPDm7sUyFZ_VVCJN/view

Evers, H. G., Pinnegar, J. K., & Taylor, M. I. (2019). Where are they all from?–sources and sustainability in the ornamental freshwater fish trade. Journal of Fish Biology, 94(6), 909-916.

Green, J., Coulthard, E., Megson, D., Norrey, J., Norrey, L., Rowntree, J. K., … & D’cruze, N. (2020). Blind trading: a literature review of research addressing the welfare of ball pythons in the exotic pet trade. Animals, 10(2), 193.

Marchio, E. A. (2018). The art of aquarium keeping communicates science and conservation. Frontiers in Communication, 3, 17.

Muka, S. (2022). Taking hobbyists seriously: The reef tank hobby and knowledge production in serious leisure. Studies in History and Philosophy of Science, 93, 192-202.

Nekaris, K. A. I., Musing, L., Vazquez, A. G., & Donati, G. (2015). Is tickling torture? Assessing welfare towards slow lorises (Nycticebus spp.) within Web 2.0 videos. Folia Primatologica, 86(6), 534-551.

Ng, T. T., Sung, Y. Y., Danish-Daniel, M., Sorgeloos, P., de Peer, Y. V., Wong, L. L., & Tan, M. P. (2021). Genetic variation of domesticated discus (Symphysodon spp.). Aquaculture, Aquarium, Conservation & Legislation, 14(2), 832-840.

Pountney, S. M. (2023). Survey indicates large proportion of fishkeeping hobbyists engaged in producing ornamental fish. Aquaculture Reports, 29, 101503.

Walster, C., Rasidi, E., Saint-Erne, N., & Loh, R. (2015). The welfare of ornamental fish in the home aquarium. Companion Animal, 20(5), 302-306.

The perfect small ‘eel’, Macrognathus circumcintus a short guide

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Many fishkeepers are after something unique and usual for their aquarium and sadly many of what we call oddballs are usually quite large. There are always exceptions to this and these only need to be encouraged.

The eel issue

Before exploring Macrognathus circumcinctus I should explain the issue with the word eel. Eel is a common name that realistically describes anything anguilliform in shape. Anguilliform is a fish shape which refers to that elongate body shape and would generally use that body for locomotion. This is easily confused with Anguilliformes, true eels are quite distantly related from what we see sold as freshwater eels with the exception of the large Gymnothorax polyuranodon (Lifestyle is debated; Ebner et al., 2011; Ebner et al., 2019) and G. tile (actually brackish). Eels are found in many clades from the eel loaches in Pangio and Vaillentella; to the eel catfishes in Channallabes. Gymnotiformes, true knifefishes are also described as either eels or having anguilliform morphology.

No group is referred to as eels as much as Synbranchiformes and this is where Macrognathus circumcintus places. Other popular ‘eel’ fishes are also found in this clade, Mastacembelus erythrotaenia (Fire eel), Mastacembelus armatus (tyre track eel) and if more interested in Asian cooking then Monopterus spp. (swamp eels). The South American swamp eel, Synbranchus spp. is particularly interesting given along with being obligate air breathers they are sequential/diandric hermaphrodites (Barros et al., 2017).

The Half Banded Spiny Eel, Macrognathus circumcinctus

Taxonomy: Previously Mastacembelus before being revised in 1984.

Locality: Widespread throughout South East Asia. GBIF (Global Biodiversity Information Facility) records this species largely in Thailand and Cambodia with a few reaching out into Vietnam, Malaysia, Borneo and Sumatra. Whether these represent the same species I don’t know.

Size: 9-13cm SL (Maharsi et al., 2024; Parenti & Lim, 2005), there is some report of a 28.5cm individual from Fishbase but this seems exceptional.

Habitat: It seems they prefer rocky habitats with reasonable currents (Maharsi et al., 2024), they prefer it shaded and it also seems to be largely turbid not by tannins but by sediment. These habitats probably have a lot of invertebrates feeding on the diversity of bacteria.

Temperature: 26.7-29.2 (Maharsi et al., 2024)

pH: 6.4-8.33 (Maharsi et al., 2024).

Diet: Invertebrates (Maharsi et al., 2024), maybe small fishes but I have seen mine actively feed on mushrooms. They can be quite quick ambush predators while also capable of searching for food amongst crevices. This invertebrate diet is pretty consistent throughout Macrognathus (Sarmah & Kardong, 2024). Ideally as a fishkeeper it would be worth aiming for a diet with an insect/invertebrate based dry/gel diet such as Repashy Bottom Scratcher Or Fluval Bug Bites. Building on that diet with a diversity of frozen and freeze dried foods. You can also try live foods such as earthworms on larger individuals but tubifex, blackworms, daphina etc. on smaller.

Hardscape: A lot of decor so they have places to hide and explore, a confident individual will come out but also if in a group it gives them space to hide should be required.

Substrate: While I’ve never experienced this species burying particularly and even less with age many will bury in the sand and it is therefore beneficial as enrichment to provide that for them.

Sociality, a problem?

While almost all websites on the species will record this species as social, it doesn’t take much to find out the picture is a little more complicated. In reality some are and it’s worth buying more if you can because if yours are social they will benefit from that interaction as basic enrichment. Mine and many others have M. circumcinctus who wont tolerate others of their species and even other eel shaped fishes, while I thought it was because even before I took on mine it was housed alone for potentially many years. Over time it’s become clear mine is not alone and they can even be aggressive from a younger age. They are very much individuals so it is likely that more then anything else.

References:

Barros, N. H. C., De Souza, A. A., Peebles, E. B., & Chellappa, S. (2017). Dynamics of sex reversal in the marbled swamp eel (Synbranchus marmoratus Bloch, 1795), a diandric hermaphrodite from Marechal Dutra Reservoir, northeastern Brazil. Journal of Applied Ichthyology, 33(3), 443-449.

Ebner, B. C., Donaldson, J. A., Courtney, R., Fitzpatrick, R., Starrs, D., Fletcher, C. S., & Seymour, J. (2019). Averting danger under the bridge: video confirms that adult small-toothed morays tolerate salinity before and during tidal influx. Pacific Conservation Biology, 26(2), 182-189.

Ebner, B. C., Kroll, B., Godfrey, P., Thuesen, P. A., Vallance, T., Pusey, B., … & Perna, C. N. (2011). Is the elusive Gymnothorax polyuranodon really a freshwater moray?. Journal of Fish Biology, 79(1), 70-79.

Fricke, R., Eschmeyer, W. N. & Van der Laan, R. (eds) 2024.  ESCHMEYER’S CATALOG OF FISHES: GENERA, SPECIES, REFERENCES. (http://researcharchive.calacademy.org/research/ichthyology/catalog/fishcatmain.asp). Electronic version 05/09/2024.

Maharsi, G. J., Waluyo, W., & Armando, E. (2024). Study of Ecobiological Characteristics of Spiny Eel (Mastacembelidae) in Elo River, Magelang, Central Java. JFMR (Journal of Fisheries and Marine Research), 8(3), 8-15.

Parenti, L. R., & Lim, K. K. (2005). Fishes of the Rajang Basin, Sarawak, Malaysia. The Raffles Bulletin of Zoology, 13, 175-208.

Sarmah, P., & Kardong, D. (2024). Biology Of Macrognathus Sp. With Special Reference To Macrognathus Pancalus (Hamilton) And Macrognathus Aral (Bloch And Schneider). Journal of Advanced Zoology, 45(1).

Are nitrates harmful to aquarium fishes?

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It is quite common for nitrates to be labelled as non-toxic to fishes and it is kind of accepted that nitrates have a lower toxicity to ammonia and nitrites, or more that these have a higher acute (short term) toxicity. Every aquarium community will give you a different maximum level of nitrate toxicity, this kind of reflects that nitrate toxicity is very much misunderstood and rarely fact checked. Not just is this topic rarely fact checked but often it is a lot more complex as it takes understanding the limitations of the scientific literature.

One of the beautiful angelfish, Pterophyllum scalare at the Keystone Clash 2024.

It is noted in the scientific literature that nitrate exposure is neglected (Monsees et al., 2017), so why do we focus on the lack of studies in the assumption it’s non-toxic with little science to say it is.

At the end of the day, we know nitrates is toxic. We don’t know the safe levels and therefore it is more ethical to assume that as little as possible is safer then more because we know that nitrates have no benefits. It is better to assume that negative for the sake of a little bit of hard work if it could benefit the animal.

There are definitely flaws in the idea nitrates are not toxic particularly when saying up to 20, 40, 100ppm are fine. Let me discuss each limitation to these claims:

  • Most studies focus on species that we do not keep, hardy species such as tilapia (Oreochromis; Monsees et al., 2017), carp, Salmonids (Yu et al., 2021) or Danio rerio. Many of these do not apply either due to temperature or just not closely related to anything we are discussing.
  • These studies are short term so look at acute exposure not long term chronic exposure (Monsees et al., 2017; Yu et al., 2021). If the studies only look at 30 days it cannot be assumed that the effect is the same at 5, 10 or 20 years.
  • Studies use distinct gradings of exposure, often wide values so it is not possible to understand effects between those values (Gomez Isaza & Cramp, 2020; 2021; Yu et al., 2021). If they only test from 10ppm onwards the effect below 10ppm is unknown (Cano-Rocabayera et al., 2019) and if effects are only noted at 50ppm but they don’t test between that and 10ppm it cannot be assumed that 50ppm is a minimum level.
  • No study looks at all effects of exposure, particularly physiological effects so could miss other effects of the treatments exposed (Cano-Rocabayera, 2019; Monsees et al., 2017; Yu et al., 2021).
  • Many people focus on lethal effects in the short term and lethal doesn’t mean it isn’t causing other damage.

Why these limitations? Well the funding understandably is highest for food fishes and these do not need to be kept long term particularly on the scale for fishkeeping, along with that the aim is often not reproduction.

Lasiancistrus sp. obtained from Acres Aquatics, Warminster UK.

We know that nitrate is a pollutant that requires long term, chronic exposure (Yu et al., 2021) so this is where the limitation in understanding arises. When it comes to exposure levels 0ppm is usually the control to the next exposure level is tested there are effects on the fishes physiology (Kellock et al., 2018; Gomez Isaza & Cramp, 2021) that many aquarists would not be able to test for. While not always statistical there seems to always be effect but not lethal till very very high concentrations e.g. 600ppm, Regardless it does effect adaptability to change (Presa et al., 2022).

It does seem in every study there is a difference between low level exposure and no exposure in the short term in those cited but it is not significant, what about long term? If we think this is short term exposure causing some damage but over time and years, is recovery possible? I’d assume likely not.

There is never really any support for any one value to be the toxic level for many or any fishes (Isaza et al., 2020; Presa et al., 2011; Villa-Villaseñor et al., 2022; in general any of the citations).

What are the effects of nitrates?

Natural habitats are known as oligotrophic and normally have very little nitrates that is close to the undetectable level without a pollution event. Clearly captive fishes have a little more adaptability but many of the fishes we keep are wild caught and does it depend on the generation (Isaza et al., 2020).

Male red cover discus, Symphysodon sp. x. with Xiphophorus maculatus x. X. variatus

Test kit reliability.

The crutch a reliance on nitrates being low levels before water changes relies on reliable test kits. Dip stick, paper test kits have always been argued as unreliable, giving both false negatives and false positives although when first opened many will note they do show some reliability. Liquid test kits are a little bit more complicated, obviously we can’t calibrate them personally as we have nothing to calibrate them against as a hobby and similarly if using a digital system.

We think our liquid test kits are 100% reliable but they do expire and once opened it’s tricky to say when this happens. I generally say liquid test kits should be replaced annually as there can be false results after that depending on brand and product. But I have seen some test kits newly opened, long before their expiry date give both false negatives and positives for nitrates. So, why rely on these for water changes?

Conclusions

I think this argument is similar to UVB to produce vitamin D in reptiles, there has always been the idea that UVB is not needed for certain reptiles, they are nocturnal or consume whole mammals who provide vitamin D. Yet over time there has been evidence that even in these animals there is a benefit to providing Vitamin D. As this is not a reptile website and to save all the citations I will forward the Reptile Lighting Facebook group: https://www.facebook.com/groups/384134861721116 This group provides many citations.

Water changes are vital but also better to be cautious when it comes to test kits and go on the precaution of maybe more nitrates then you actually have. It’s also worth testing your tap water.

While we don’t know the long term damage of low level nitrates it’s worth acting on the precaution.

References:

Cano-Rocabayera, O., De Sostoa, A., Padrós, F., Cardenas, L., & Maceda-Veiga, A. (2019). Ecologically relevant biomarkers reveal that chronic effects of nitrate depend on sex and life stage in the invasive fish Gambusia holbrooki. Plos one, 14(1), e0211389.

Gomez Isaza, D. F., Cramp, R. L., & Franklin, C. E. (2020). Simultaneous exposure to nitrate and low pH reduces the blood oxygen-carrying capacity and functional performance of a freshwater fish. Conservation physiology, 8(1), coz092.

Gomez Isaza, D. F., Cramp, R. L., & Franklin, C. E. (2021). Exposure to nitrate increases susceptibility to hypoxia in fish. Physiological and Biochemical Zoology, 94(2), 124-142.

Isaza, D. F. G., Cramp, R. L., & Franklin, C. E. (2020). Living in polluted waters: A meta-analysis of the effects of nitrate and interactions with other environmental stressors on freshwater taxa. Environmental Pollution, 261, 114091.

Kellock, K. A., Moore, A. P., & Bringolf, R. B. (2018). Chronic nitrate exposure alters reproductive physiology in fathead minnows. Environmental Pollution, 232, 322-328.

Monsees, H., Klatt, L., Kloas, W., & Wuertz, S. (2017). Chronic exposure to nitrate significantly reduces growth and affects the health status of juvenile Nile tilapia (Oreochromis niloticus L.) in recirculating aquaculture systems. Aquaculture Research, 48(7), 3482-3492.

Presa, L. S., Neves, G. C., Maltez, L. C., Sampaio, L. A., Monserrat, J. M., Copatti, C. E., & Garcia, L. (2022). Acute and sub‐lethal effects of nitrate on haematological and oxidative stress parameters of juvenile mullet (Mugil liza) in freshwater. Aquaculture Research, 53(9), 3346-3357.

Villa-Villaseñor, I. M., Yáñez-Rivera, B., Rueda-Jasso, R. A., Herrera-Vargas, M. A., Hernández-Morales, R., Meléndez-Herrera, E., & Domínguez-Domínguez, O. (2022). Differential sensitivity of offspring from four species of goodeine freshwater fish to acute exposure to nitrates. Frontiers in Ecology and Evolution, 10, 1014814.

Yu, J., Xiao, Y., Wang, Y., Xu, S., Zhou, L., Li, J., & Li, X. (2021). Chronic nitrate exposure cause alteration of blood physiological parameters, redox status and apoptosis of juvenile turbot (Scophthalmus maximus). Environmental Pollution, 283, 117103.

INKBIRD  ITC-306A Review and Setup

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INKBIRD is a brand that produces external thermostat, these are units that regulate the temperature change provided by a heating unit. Without a thermostat the heater will not switch off at the desired temperature or switch on when the temperature is too low. Many heaters contain their own ones but very frequently they do malfunction, these external units are more reliable. Some heaters do not have an internal thermostat such as many titanium products so would require an external thermostat to avoid overheating the tank.

There aren’t many external thermostats on the market other than INKBIRD’s temperature controllers. I will be discussing the INKBIRD  ITC-306A as this is the one I currently use and it has all of the features I discuss. These external thermostats by INKBIRD are also unique as being controlled via an app that you can view/control remotely so any issues with temperature can be controlled wherever you are. The reptile hobby does have a wider range of thermostats but the probes used to measure the temperature are often not waterproof and do not have an app to control the temperatures.

Generally the majority of people set their external thermostats as the only thermostat with the heater unit itself at a much higher temperature. Personally I prefer to use both as a fail safe, so the heaters are only setup above that of the INKBIRD temperature controller but within a tolerable range for the fishes. So my desired range is 28.5-29c, it’s what I set the INKBIRD temperature controllers to but I set the heaters to around 32c which is tolerable for the fishes.

The other benefit is these are connected to WIFI and therefore can be controlled from my phone. On top of that it will send push alerts for any issues and if I notice that the units are disconnected it could hint to a power or wifi outage.

These thermostats do not require only to be controlled via your phone but also can be used manually if that is preferred. It also means they will stay running during any WIFI or power outage. The temperature history also allows for you to loosely track how long it takes to heat the tank and also if and when there was any power outages.

One feature that is not really discussed much within fishkeeping is due to the app’s record of temperature changes and the long heating alarm you can identify how the heaters are managing to heat up the tank. Aquarium heaters usually don’t last more then a few years and over time particularly at higher temperatures they struggle to reach the desired temperature or reach that temperature rapidly. You can identify this with the heating record as the heaters take longer to heat the tank or the long heating alarm will set off after a desired amount of time, I usually go for around 24 hours as if a heater takes longer then that I need to replace it.

All alarms can also be switched off remotely which is great!

Review/opinion:

I have two INKBIRD temperature controllers myself and have done since 2021, they have proven reliable to me and a necessary part of my aquariums since having heaters fail in the past. They are really good and reliable units, setting them up is quick and they have a memory so you don’t need to change any settings every time you attach your app to each unit. Where I do keep fishes that require 28c I think it puts a lot of pressure on most heaters so the INKBIRD thermostats really help here. I have had heaters overheat a tank before or just stop working so without an INKBIRD I had no idea till I checked the thermometer. Since having the INKBIRD temperature controllers they are a much more reliable method of controlling the temperature. The thermostat allows for two heaters to be attached meaning it doesn’t take as long to reach the desired temperature. I really find the app useful as well, anything remote saves me worrying about my fishes.

Setting up your INKBIRD

Step 1: Download the INKBIRD app on the Apple App store or what is required for Android.

Step 2: Your home screen should look similar to this without any INKBIRD temperature controllers setup. Click on the Plus Icon.

Step 2: Choose the product you have, here the INKBIRD-306A.

Step 3: Choose the mode. I find AP mode the easiest, it might not be automatically set to that so press the other mode option to select it.

Step 4: Press the WIFI button on the INKBIRD until the light on the unit blinks slowly. This might take several goes.

Step 5: The INKBIRD will then require your WIFI including password of which needs filling in.

Step 6: It will request for you to join the network that is provided by the INKBIRD unit. Go onto your WIFI settings and join that network.

Step 7: Press connect.

Step 8: It will then connect which can take time, keep the phone close to the INKBIRD unit while connecting.

Step 9: All connected, you can name the unit.

Running an INKBIRD

This is the main screen, it has several updates so hadn’t always looked like this. It should be pretty self explanatory how to use.

The central value is the temperature where it will tell you if the heaters are providing heat or not. This screen is also where any warnings will appear such as low temperature, wide temperature range between probes etc.

You can select temperature by pressing on that range T1-T2.

Under settings you will find this screen, this is where you can select alarms. I have mine currently at around 26c so I know if the temperature drops below the tolerable range for my fishes long term. Heating time might also be worth considering, it’ll produce an alarm if too long so consider how long after a water change the aquarium takes to reach it’s desired temperature.

Video Review

How to Sex your Loricariid (Pleco/whiptail catfish).

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This is a big question when it comes to keeping Loricariids in the aquarium as more then often people want to spawn the fishes but sometimes people just want to name them.

What is the sex determination methods in Loricariidae?

This is actually a more vast discussion then some other groups like mammals. In fact Loricariid’s use a range of karyotypes to determine sex from the more well known ZW and XY to multiple sex chromosomes. Ancistrus (Bristlenoses) is a large genus representing over 60 spp. (Eschmeyer, 2025), this genus does show multiple sex determination methods and some not as well known such as ZZ/ZW1W2 in Ancistrus clementinae (Nirchio et al., 2023) or ZZ/ZW in Ancistrus ranunculus (Oliveira et al., 2007). It seems many genera show a lot of diversity (Sassi et al., 2023) and this is possibly a large barrier to hybridization and maybe could lead to speciation.

What do you need to be able to identify the sex of your Loricariid?

  • A mature fish, unless the fish has reached sexual maturity it likely wont show many sexually dimorphic features although there are slight exceptions at the smaller level or during dissection. Largely this will mean close to fully grown and/or clearer odontode growth. Some species this might take a year such as some of the more common Ancistrus or 5+ years like the Hemiancistrus medians group (e.g. Panaque, Baryancistrus etc.)
  • A bowl, container or polybox can be useful to examine around the fish particularly below. Fill this with the tank water, do not do it during acclimation. It can be done in store.

Anatomy that allows you to sex Loricariids

Venting – Genital papilla and the urogenital pore

Figure 1: Baryancistrus chrysolomus, mango/magnum pleco juvenile.

The genital papilla and the urogenital pore are the same thing, this is a combined organ where the fish passes waste but also the gametes (eggs and sperm). This is not the same for all Loricariids where the genital pore is separate from the anus such as in Neoplecostomus. It is clearly on the abdomen, shape is normally ambiguous in juveniles.

The shape of the genital papilla is the most reliable method to sex Lorcariids, it’s most easily stated as V shaped in males and U in females. This can be tricky to see in some genera which have more elongate genital papilla such as the Pterygoplichthys in figure 2.

Figure 2: created by In the Bag Tropical Fish UK, Alice Cook. Depicting Pterygoplichthys pardalis and Pterygoplichthys gibbiceps.
Figure 4: Mature female Baryancistrus chrysolomus, not showing the clearest maturity.

Some other genera the females are much wider so it is much more square as above. The best method is to see multiple individuals and compare between them which can be done against the glass and taking photos or even within a container to take a more up close look.

Figure 5: The genital papilla of Chaetostoma as explained by Lujan et al. (2015) in Lujan, N. K., Meza-Vargas, V., Astudillo-Clavijo, V., Barriga-Salazar, R., & López-Fernández, H. (2015). A multilocus molecular phylogeny for Chaetostoma clade genera and species with a review of Chaetostoma (Siluriformes: Loricariidae) from the Central Andes. Copeia, 103(3), 664-701.

I find this method the most reliable particularly as there is less individual variation between individuals and also depends less on the age of the fish. The shape can be applied also to Loricariinae (whiptail catfishes) and Hypoptopominae (Otocinclus, Parotocinclus, Hypoptopoma etc.).

The final part of the genital papilla when it comes to sexing Loricariids is spotting around this region in females. These papilla/spots can be green or yellow in colour and can range from one to many in number, once visible to the eye it hints maturity in the females. While it is very obvious in Hypostominae (traditional pleco’s) it is unclear as to if these are present in the other subfamilies even the distantly related but similarly looking Pseudancistrus genisetiger (slate pleco’s) and Rhinelepinae. Although I have been told by Fauna Tropica (https://www.faunatropica.eu/) that these spots can be seen under a microscope and maybe a macrolens before maturity even as younger juveniles.

Figure 6: Genital papilla of a female Baryancistrus chrysolomus.

Body shape

I am not a massive fan of using body shape as it can depend on many factors. Using how plump the fish is does depend on how well the fish has been fed but also females once the female has released eggs can suddenly reduce weight.

Head shape does seem the most reliable method regarding the general anatomy but can be limited when it comes to stunted fish but there is also a lot of individual variation. In general it is assumed females display more elongate heads whereas males are shorter and wider, this is a trend we do see in other fishes.

Figure 7: Body shape in two different Scobinancistrus species but clearly shows the sexual dimorphism.

Personally when it comes to sexing individuals from photos I am less of a fan of this method as it does rely even more on angle of the photo but also maturity. You can see a larger difference in some genera then others and some species you might not see it at all.

Odontodes

Figure 8: Opercular odontodes on Ancistrus ranunculus.

Odontodes are the external teeth that cover Loricariids, this does include everything from Otocinclus to Farlowella to Hypancistrus. In some species they can be enlarged in certain areas and this is known as hypertrophied, further in some these odontodes can be sexually dimorphic. Odontodes as a secondary sexual characteristic though are not a rule and there are many exceptions, in addition they can be seasonal so males might have reduced ones seasonally and females can have larger ones, it depends on the genus.

Figure 9: Panaqolus albivermis (flash pleco), male showing clear hypertrophied odontodes.

Odontodes are most useful in the Peckoltia clade, this includes Hypancistrus, Peckoltia, Panaqolus, Pseudoqolus, Ancistomus and Scobinancistrus. Although Scobinancistrus and species such as Peckoltia sabaji do not have particularly sexually dimorphic odontodes and I recommend using the genital papilla. These odontodes are hypertrophied in all individuals at the gill opercular although can be larger in males. The most obvious method with this clade is hypertrophied odontodes on the caudal peduncle and pectoral fin of the males.

Figure 10: Baryancistrus demantoides (high finned green phantom), female.

When using this trait be careful with other clades, some such as the Hemiancistrus medians group which includes Hemiancistrus medians, some other Hemiancistrus (this genus requires revision), Panaque, Parancistrus, Baryancistrus and some Spectracanthicus (again another genus that requires revision). In these clades I find females can grow large odontodes seasonally and this was very evident to me in a clearly female Baryancistrus demantoides (Fig 10). The key thing to note about odontodes is they can be shed.

Figure 11: Farlowella vittata group pictured at Aqualife, Leyland.

Another clade that easily gets forgotten is Loricariinae, while genital papilla are a clear way to sex many genera within this subfamily. Odontodes can provide an additional quick way to sex many particularly Farlowellini (Farlowella, Sturisoma and Sturisomatichthys mostly; Fig 11). These are reliably hypertrophied around the head and/or rostrum in this clade.

Tentacles

This is an exclusive trait to the genus Ancistrus and the species Lasiancistrus tentaculatus. These are fleshy growths derived from the odontode sheaths (Sabaj et al., 1999), which might explain why sexually dimorphism is shown as related clades such as Lasiancistrus shows some clearer dimorphism in the odontodes. In Ancistrus while in the common bristlenose the males display larger tentacles whereas the females display little to none, there are Ancistrus where females have large tentacles, some which have none and some where the size is the same. Not to be confused with odontodes which is why it is best not to refer to them as spines or bristles.

Some factors that are often myths when it comes to sexing Loricariids

Behaviour

It is a common misconception that males are more aggressive then females and often this has no grounding in experience or science, it’s often an assumption. Males and females do have different territories as males are involved in the brood care whereas females roam but where species are territorial it is in both sexes. In territorial species aggression is shown between and within the sexes. Often this idea of males being more aggressive is based in the coy female myth (Milam, 2013; Rosvall, 2013), many scientists have disproved this but Lucy Cooke makes some good approachable books to the topic. Females in Loricariids have no reason to be less aggressive. Personal experience I’ve seen aggression from both and if anything where females tend to roam their aggression is wider spread then close to the caves where males dwell.

The other myth is that females will not use caves, particularly as juveniles they will definitely hide a lot but even as adults females use caves as refuges, they might even have a preferred cave. Males use the caves to spawn in Hypostominae and some Loricariinae but not all do and those that like crevices will use them.

Conclusion

It can be really tricky to identify whether your Loricariid is male or female. Hopefully this offers some ideas to help sex your fishes. I tend to recommend using the abdomen of the fish as I feel this is most reliable and doesn’t leave space for any amounts of individual variation.

For great comparative photos check out: https://www.suedamerikafans.de/en/zur-unterscheidung-der-geschlechter-klein-bleibender-hypostominaer-harnischwelse/

References:

Lujan, N. K., Meza-Vargas, V., Astudillo-Clavijo, V., Barriga-Salazar, R., & López-Fernández, H. (2015). A multilocus molecular phylogeny for Chaetostoma clade genera and species with a review of Chaetostoma (Siluriformes: Loricariidae) from the Central Andes. Copeia, 103(3), 664-701.

Milam, E. L. (2013). Making males aggressive and females coy: Gender across the animal-human boundary. In Women, Science, and Technology (pp. 206-222). Routledge.

Nirchio, M., Oliveira, C., de Bello Cioffi, M., de Menezes Cavalcante Sassi, F., Valdiviezo, J., Paim, F. G., … & Rossi, A. R. (2023). Occurrence of sex chromosomes in fish of the genus Ancistrus with a new description of multiple sex chromosomes in the Ecuadorian endemic Ancistrus clementinae (Loricariidae). Genes, 14(2), 306.

Oliveira, R. R. D., Feldberg, E., Anjos, M. B. D., & Zuanon, J. (2007). Karyotype characterization and ZZ/ZW sex chromosome heteromorphism in two species of the catfish genus Ancistrus Kner, 1854 (Siluriformes: Loricariidae) from the Amazon basin. Neotropical Ichthyology, 5, 301-306.

Rosvall, K. A. (2013). Proximate perspectives on the evolution of female aggression: good for the gander, good for the goose?. Philosophical Transactions of the Royal Society B: Biological Sciences, 368(1631), 20130083.

Sabaj, M. H., Armbruster, J. W., & Page, L. M. (1999). Spawning in Ancistrus (Siluriformes: Loricariidae) with comments on the evolution of snout tentacles as a novel reproductive strategy: larval mimicry. Ichthyological Exploration of Freshwaters, 10(3), 217-229.

Sassi, F. D. M. C., Deon, G. A., Sember, A., Liehr, T., Oyakawa, O. T., Moreira Filho, O., … & Cioffi, M. D. B. (2023). Turnover of multiple sex chromosomes in Harttia catfish (Siluriformes, Loricariidae): a glimpse from whole chromosome painting. Frontiers in Genetics, 14, 1226222.

Bee’s, a benefit to aquariums?

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The use of various items produced by bees is popular within our daily lives, the most commonly used product being honey. The health benefit of products related to bees have been noticed for many hundreds maybe thousands of years from anti-microbial properties to general health. Honey maybe isn’t the most practical in the aquarium but two products you might find; bee pollen and propolis.

What is bee pollen?

Bee pollen is literally just the pollen collected by the bees. It is generally collected by forcing the bees to enter the hive through smaller then normal holes so the pollen drops below to be collected. This possibly puts a lot of pressure on the hive depending on how this is done.

What is the purpose of bee pollen in the aquarium?

Bee pollen is a dietary additive, there is a rising popularity in regards of this ingredient for people and perhaps this is the source of the idea. While it is an interesting ingredient and for those that might feed on fruit there could be a benefit. The actual benefits in general are debatable, with an omnivorous/carnivorous Clarias sp. catfish there were many physiological benefits shown up to 1% addition (Nowosad et al., 2022). When bee pollen was added to the diet of the insectivore zebrafish, Danio rerio there is no shown benefits to physiology but improve viral resistance (Di Chiacchio et al., 2021). In Nile tilapia, Oreochromis niloticus liver regeneration is shown to occur when fed bee pollen (Picoli et al., 2019).

What is propolis?

Propolis is the compounds and structures that seal the hive and the waxy structures that the hive is built on. It originates from the waxy areas of plants such as buds.

What is the purpose of propolis in the aquarium?

It’s not particularly common in aquariums but you might find the produce available more within the koi hobby. This product is used largely to seal wounds not just for protection against microbes entering them but also if you want to seal in a treatment. There does seem to be little research into the topic although some of the compounds do show promise when it comes to wound healing (Wibowo et al., 2021) and nutrition (Farag et al., 2021). More importantly there seems to be no papers onto any toxicity of propolis which is somewhat promising but could also be useful to cross out.

Conclusion

A very brief article I guess as to what could apply to the aquarist, it seems there needs a lot more research but I wouldn’t be afraid to use it. Personally I think bee pollen could be great for promoting feeding in fishes due to the sugars present. It’s something I think we could explore further.

References:

Di Chiacchio, I. M., Paiva, I. M., de Abreu, D. J., Carvalho, E. E., Martínez, P. J., Carvalho, S. M., … & Murgas, L. D. S. (2021). Bee pollen as a dietary supplement for fish: Effect on the reproductive performance of zebrafish and the immunological response of their offspring. Fish & Shellfish Immunology, 119, 300-307.

Farag, M. R., Abdelnour, S. A., Patra, A. K., Dhama, K., Dawood, M. A., Elnesr, S. S., & Alagawany, M. (2021). Propolis: Properties and composition, health benefits and applications in fish nutrition. Fish & Shellfish Immunology, 115, 179-188.

Nowosad, J., Jasiński, S., Arciuch-Rutkowska, M., Abdel-Latif, H. M., Wróbel, M., Mikiewicz, M., … & Kucharczyk, D. (2022). Effects of bee pollen on growth performance, intestinal microbiota and histomorphometry in African catfish. Animals, 13(1), 132.

Picoli, F., Lopes, D. L. D. A., Zampar, A., Serafini, S., Freccia, A., Veronezi, L. O., … & Emerenciano, M. G. C. (2019). Dietary bee pollen affects hepatic–intestinal histomorphometry of Nile tilapia fingerlings. Aquaculture Research, 50(11), 3295-3304.

Wibowo, I., Utami, N., Anggraeni, T., Barlian, A., Putra, R. E., Indriani, A. D., … & Ekawardhani, S. (2021). Propolis can improve caudal fin regeneration in zebrafish (Danio rerio) induced by the combined administration of Alloxan and glucose. Zebrafish, 18(4), 274-281.