Many fishes display lumps, spots and bumps which can happen due to a range of pathological conditions whether it being pathogens or other factors. These can easily be confused and if concerned or confused please consult a fish pathologist or trained fish specialist veterinarian. This page is only to help give some clarity

All sources here are from scientific papers and many of the images from these papers for a reliable diagnosis unless otherwise stated for clarity purposes.

Microscopy is required in many cases for a confirmed diagnosis and therefore I recommend their use. Some stores might have one available to be used. In other cases more advanced diagnostic techniques might be required and provided by a pathologist via a veterinarian.

Diseases featured:

1. White spot/ich (Ichthyophthirius multifiliis)
2. Velvet (Oodinium, Piscinoodinium and other dinospores) and related taxa
3. Dermocystidium
4. Worm cysts
5. Tumors
6. Viral papilloma and herpes viruses
7. Unknown growths

White spot/ich (Ichthyophthirius multifiliis)

Protozoan ciliate pathogen which can cause large mortalities in later stages.

Occurrence: Seems to be very common and in my opinion largely occurs when fish are sufficiently stressed.

Diagnostics: White spots on the body of the fish, can also be accompanied by shedding of the slime coat. The spots can be varied in size and number. This ciliate additionally targets the gills where it causes the most stress on the fish (Yang et al., 2023; Mallik et al., 2013). This spotted appearance is commonly confused with Epistylis which if symptomatic (of which mostly it is not) appears more as a plaque so will not appear in this article (ESHA video; Ksepka et al., 2021; Valladao et al., 2015; Wang et al., 2017, Wu et al., 2021). Both pathogens can appear with Aeoromonas bacteria that additionally causes hemorrhaging (Kumar et al., 2022).

Microscopic image:

Treatment: The most frequent treatments provided in the aquarium hobby is malachite green with either formalin, formaldehyde or copper. Obviously copper is often a concern for those with invertebrates in the aquarium. Salt the old age treatment can work and as a study ABDULLAH-AL MAMUN et al. (2021) infers it depends on time of treatment. There seems to be a multitude of papers narrowing down the best treatment and to me this infers maybe there is no real answer, there might be increasing immunity of the ciliate or just this species is so diverse. Generally it’s assumed the best method is to avoid introducing any fish showing symptoms or on the same system with those who do. Copper sulphate additionally has been shown to treat white spot (Schlenk et al., 1998). In a later 2008 study formalin did not have any effect on the ciliate whereas copper treated it within 14 days (Rowland et al., 2008) which is around two rounds of treatment by most bottles. I therefore just looking at these three studies would not recommend formalin and maybe therefor formaldehyde for treatment and instead focus on copper and maybe salt needs further examination.

Velvet (Oodinium, Piscinoodinium and other dinospores) and related taxa

One of the most unusual parasites of fishes being an algae known as dinoflagellates.

Occurrence: Reasonably common, I can’t comment on cause. It seems Oodinium seems to be less common then Piscinoodinium.

Diagnostics: Different from white spot, being algaes they tend to have some coloration to them. Usually appears as small golden spots across the body, the fish are often lethargic. Mortality is not particularly rapid (Levy et al., 2007). It is not always obvious beyond hemorrhaging (Sudhagar et al., 2022).

Microscopic image: Not entirely required and diverse, although can make it easier to compare.

Treatment: Treatment rarely seems discussed. As these are photosynthetic I agree with many aquarists that keeping the lights off for even two weeks will not harm the fish and even if it minorly effects the algae that’s better then nothing. The aquarium hobby largely jumps to copper based treatments along with malachite green mixed in with methylene blue, it doesn’t seem the most resistant pathogen and I wonder additionally if a fishes immune system handles some of it. Copper is suggested to work (Lieke et al., 2020). There seems to be no research on methylene blue other then being an anti-protozoan compound. Similarly with malachite green, no treatments even salt have any strong background to their effectiveness. The other issue is these compounds can effect the microbes within the aquarium (Yang et al., 2021). All I can say is I had success with the NT labs anti-parasite treatment containing copper sulphate and formaldehyde along with switching the lights off.

Dermocystidium

The strangest in appearance of pathogens, much like a worm but far from that, their taxonomic placement is unclear but they don’t move.

Occurrence: Reasonably frequent with a diversity of fishes soon after obtaining.

Diagnostics: Worm like in a variety of forms and size, can be encapsulated by pectoral fins or around the body. Some might appear singularly other fishes might have a lot more. These do not move like nematodes but are extremely diverse (Fujimoto, et al., 2018; Persson et al., 2022).

Microscope image: Not required.

Treatment: It is largely agreed in the aquarium trade which I agree that these protozoans as unsightly as they are do disappear with time and handled by the fishes own immune system.

Worm cysts

Not really possible to obtain much scientific research on the topic with the time I have free. But it does happen and seems frequent with wild imports.

Occurrence: Not common at all, seems to arrive with import.

Diagnostics: Looks much like white spot but larger cysts. Stubborn and white spot treatments do not work regardless of the fish showing no lethargy or poor health.

Microscope image: Usually distinctively different from white spot velvet by size and shape.

Treatment: As suggested by the name, wormers tend to work. I cannot remember which I used but likely first praziquantel, levamisole and flubendazole might work as well.

Tumors

If a tumor occurs certainly this is a situation where veterinarians are recommended.

Occurrences: There are many caused within fishes, sometimes it is genetic causes such as in the dragon scale of Betta splendens, viral causes (Coffee et al., 2013) or dietary (Žák et al., 2022). On a quick scan of the literature papilloma viruses are not associated with tumors.

Diagnosis: These can be large growths, they can be vascularized supplied by blood vessels. Unlike viral growths on my observations tend to be rounder and do not appear in many numbers.

Microscopy: Not required.

Treatment: Discuss with a specialist fish veterinarian.

Viral papilloma and herpes viruses

These are commonly mistaken for lymphocystis and can be very complex growths. Technically Papilloma refers to epithelial groups but can display similarly to herpes viruses. In the aquarium trade it doesn’t seem clear which we are handling and regardless treatment is the same. A wide range of herpes viruses are associated with fishes from KHV, carp pox (Davison et al., 2013) and channel catfish herpesvirus (Davison et al., 1992).

Occurrence: Seems reasonably common right now. This virus I can assume once contracted is not removed from the fishes body but are suppressed by the immune system. Koi Herpes Virus (KHV) is fatal and while not wildly common any suspected occurrence might require contacting environmental authorities.

Diagnosis: These are round or very tuberous numerous growths, much more like warts while symptomatic but asymptomatic individuals and time might not display any symptoms (Tarján et al., 2022; Rahmati-Holasoo et al., 2015). Very understudied when it comes to aquarium fishes so whether we are dealing with a herpes or traditional papilloma virus is difficult to say. Rahmati-Holasoo et al. (2015) infers that most viral growths we are seeing in Loricariids is caused by a Papillomavirus, although a previous study by Hedrick et al., (1996) in carp inferred similar growths and concluded due to a herpes virus.

Treatment: Time, these viruses cannot be cured only prevention of introduction. Very few are fatal. These viruses are generally specific to closely related fishes so transfer should not be an issue.

Sources for koi and carp pox:

https://cafishvet.com/fish-health-disease/koi-pox-aka-carp-pox

https://canalrivertrust.org.uk/things-to-do/fishing/caring-for-our-fish/guide-to-fish-health/koi-herpesvirus-khv

https://marinescience.blog.gov.uk/2015/10/02/koi-herpesvirus-khv-disease-and-fisheries

Unknown growths

There is a wide range of unknown growths in fishes we can only make assumptions, veterinary professionals and vets would be worth consulting. I have seen a number which didn’t conform with any of these.

References:

ABDULLAH-AL MAMUN, M. D., NASREN, S., RATHORE, S. S., & RAHMAN, M. M. (2021). Mass infection of Ichthyophthirius multifiliis in two ornamental fish and their control measures. Annals of Biology, (2), 209-214.

Coffee, L. L., Casey, J. W., & Bowser, P. R. (2013). Pathology of tumors in fish associated with retroviruses: a review. Veterinary Pathology, 50(3), 390-403.

Davison, A. J. (1992). Channel catfish virus: a new type of herpesvirus. Virology, 186(1), 9-14.

Davison, A. J., Kurobe, T., Gatherer, D., Cunningham, C., Korf, I., Fukuda, H., … & Waltzek, T. B. (2013). Comparative genomics of carp herpesviruses. Journal of Virology, 87(5), 2908-2922.

Esmail, M. Y., Astrofsky, K. M., Lawrence, C., & Serluca, F. C. (2015). The biology and management of the zebrafish. In Laboratory animal medicine (pp. 1015-1062). Academic Press.

Fujimoto, R. Y., Couto, M. V. S., Sousa, N. C., Diniz, D. G., Diniz, J. A. P., Madi, R. R., … & Eiras, J. C. (2018). Dermocystidium sp. infection in farmed hybrid fish Colossoma macropomum× Piaractus brachypomus in Brazil. Journal of Fish Diseases, 41(3), 565-568.

Gomes, A. L. S., Costa, J. I. D., Benetton, M. L. F. D. N., Bernardino, G., & Belem-Costa, A. (2018). A fast and practical method for initial diagnosis of Piscinoodinium pillulare outbreaks: piscinootest. Ciência Rural, 48.

Hedrick, R. P., Groff, J. M., Okihiro, M. S., & McDowell, T. S. (1990). Herpesviruses detected in papillomatous skin growths of koi carp (Cyprinus carpio). Journal of Wildlife Diseases, 26(4), 578-581.

Ksepka, S. P., & Bullard, S. A. (2021). Morphology, phylogenetics and pathology of “red sore disease”(coinfection by Epistylis cf. wuhanensis and Aeromonas hydrophila) on sportfishes from reservoirs in the South‐Eastern United States. Journal of Fish Diseases, 44(5), 541-551.

Kumar, V., Das, B. K., Swain, H. S., Chowdhury, H., Roy, S., Bera, A. K., … & Behera, B. K. (2022). Outbreak of Ichthyophthirius multifiliis associated with Aeromonas hydrophila in Pangasianodon hypophthalmus: The role of turmeric oil in enhancing immunity and inducing resistance against co-infection. Frontiers in immunology, 13, 956478.

Levy, M. G., Litaker, R. W., Goldstein, R. J., Dykstra, M. J., Vandersea, M. W., & Noga, E. J. (2007). Piscinoodinium, a fish-ectoparasitic dinoflagellate, is a member of the class Dinophyceae, subclass Gymnodiniphycidae: convergent evolution with Amyloodinium. Journal of Parasitology, 93(5), 1006-1015.

Lieke, T., Meinelt, T., Hoseinifar, S. H., Pan, B., Straus, D. L., & Steinberg, C. E. (2020). Sustainable aquaculture requires environmental‐friendly treatment strategies for fish diseases. Reviews in Aquaculture, 12(2), 943-965.

Mallik, S. K., Shahi, N., Das, P., Pandey, N. N., Haldar, R. S., Kumar, B. S., & Chandra, S. (2015). Occurrence of Ichthyophthirius multifiliis (White spot) infection in snow trout, Schizothorax richardsonii (Gray) and its treatment trial in control condition. Indian Journal of Animal Research, 49(2), 227-230.

Persson, B. D., Aspán, A., Bass, D., & Axén, C. (2022). A case study of Dermotheca gasterostei (= Dermocystidium gasterostei, Elkan) isolated from three-spined stickleback (Gasterosteus aculeatus) captured in lake Vättern, Sweden. Bulletin of the European Association of Fish Pathologists.

Rahmati-Holasoo, H., Shokrpoor, S., Mousavi, H. E., & Ardeshiri, M. (2015). Concurrence of inverted-papilloma and papilloma in a gold spot pleco (Pterygoplichthys joselimaianus Weber, 1991). Journal of Applied Ichthyology, 31(3), 533-535.

Rowland, S. J., Mifsud, C., Nixon, M., Read, P., & Landos, M. (2008). Use of formalin and copper to control ichthyophthiriosis in the Australian freshwater fish silver perch (Bidyanus bidyanus Mitchell). Aquaculture research, 40(1), 44-54.

Schlenk, D., Gollon, J. L., & Griffin, B. R. (1998). Efficacy of copper sulfate for the treatment of ichthyophthiriasis in channel catfish. Journal of Aquatic Animal Health, 10(4), 390-396.

Sudhagar, A., Sundar Raj, N., Mohandas, S. P., Serin, S., Sibi, K. K., Sanil, N. K., & Raja Swaminathan, T. (2022). Outbreak of Parasitic Dinoflagellate Piscinoodinium sp. Infection in an Endangered Fish from India: Arulius Barb (Dawkinsia arulius). Pathogens, 11(11), 1350.

Tarján, Z. L., Doszpoly, A., Eszterbauer, E., & Benkő, M. (2022). Partial genetic characterisation of a novel alloherpesvirus detected by PCR in a farmed wels catfish (Silurus glanis). Acta Veterinaria Hungarica, 70(4), 321-327.

Valladao, G. M. R., Levy-Pereira, N., Viadanna, P. H. D. O., Gallani, S. U., Farias, T. H. V., & Pilarski, F. (2015). Haematology and histopathology of Nile tilapia parasitised by Epistylis sp., an emerging pathogen in South America. Bulletin of the European Association of Fish Pathologists, 35(1), 14-20.

Yang, C. W., Chang, Y. T., Hsieh, C. Y., & Chang, B. V. (2021). Effects of malachite green on the microbiomes of milkfish culture ponds. Water, 13(4), 411.

Yang, H., Tu, X., Xiao, J., Hu, J., & Gu, Z. (2023). Investigations on white spot disease reveal high genetic diversity of the fish parasite, Ichthyophthirius multifiliis (Fouquet, 1876) in China. Aquaculture, 562, 738804.

Wang, Z., Zhou, T., Guo, Q., & Gu, Z. (2017). Description of a new freshwater ciliate Epistylis wuhanensis n. sp.(Ciliophora, Peritrichia) from China, with a focus on phylogenetic relationships within family Epistylididae. Journal of Eukaryotic Microbiology, 64(3), 394-406.

Wu, T., Li, Y., Zhang, T., Hou, J., Mu, C., Warren, A., & Lu, B. (2021). Morphology and molecular phylogeny of three Epistylis species found in freshwater habitats in China, including the description of E. foissneri n. sp.(Ciliophora, Peritrichia). European Journal of Protistology, 78, 125767.

Žák, J., Roy, K., Dyková, I., Mráz, J., & Reichard, M. (2022). Starter feed for carnivorous species as a practical replacement of bloodworms for a vertebrate model organism in ageing, the turquoise killifish Nothobranchius furzeri. Journal of Fish Biology, 100(4), 894-908.

Like any other organism fishes can host pathogens and contract diseases, unlike many other pet related hobbies we are dealing with a wide diversity of different species. The diseases of aquarium fishes is there for a vast topic.

As my specialty is in the evolutionary biology of fishes, diseases are not my specialty. That is not to say like any other fish biologist I do not encounter them, given I am often not using high magnifications I actually am more familiar with larger ectoparasites then bacteria or protozoa. I do have a background in taxonomy as well so am familiar with understanding how even these bacteria, protozoa, fungi etc. are diagnosed to a degree.

Previously there have been many brilliant aquarium books focused on disease and I think more then anything I recommend an aquarist to have at least one. These books were often written by those with a background in pathology, included images of microscope slides and based in science.

Books I’d recommend (there will be others I have forgotten):

• Discus Health, TFH Publication by Dieter Untergasser
• Handbook of Fish Diseases, TFH Publication by Dieter Untergasser.
• The Interpet Manual of Fish Health: Everything You Need to Know About Aquarium Fish, Their Environment and Disease Prevention, Firefly Books Ltd by Dr. Chris Andrews, Adrian Excell and Dr Neville Carrington.
• The Practical Guide to Fish Diseases by Dr. Gerald Bassleer

The main issue with disease is to take anything also marketing a product with a pinch of salt unless it’s discussing a general compound e.g. malachite green, formaldehyde etc.

Diseases of aquarium fishes has become one of the most hotly contested topics within the aquarium hobby of recent years. With many websites making unfounded claims that lack citations where maybe they should to backup any novel ideas or information. This makes diagnosing disease for the general fishkeeper rather challenging. I hope I can make it a little easier or give areas to start researching.

This topic will cover multiple articles and therefore this contents should help guide a user through this page and others.

1. Preventative care to avoid disease
2. What are pathogens?
3. Disease and pathogen specificity.
4. Notes on Antibiotics
5. Pathogens of fishes and relevant articles to aid in diagnosis:
6. References

Preventative care to avoid disease

One of the biggest factors when it comes to any organism of preventing disease is ensuring the organism is healthy. There are several influences that might mean a fish is stressed enough to become more susceptible to disease; behavioral, environmental and also diet. If an organism is stressed then the immune system might not be functioning as well as it could and will be less able to fend of disease, this isn’t something new. Some disease might additionally be directly caused by these factors rather then by external pathogens.

So, lets explore preventative care.

Preventing disease by understanding specific fish behavior.

This is probably the most difficult to identify and sometimes the most difficult to cater for as we know so little about fish behavior. Here there are probably two outcomes of not catering for a fishes behavior; poor physiological health or abnormal behavior.

Poor physiological health is very obvious, a fish might have tattered fins, lost scales and reduced colouration. It could be that the fish is being directly attacked in territorial aggression or from a more predatory tankmate. In some fishes just simply removing an offending individual might solve the issue but with some fishes such as the Aulonocara (peacock cichlids), mbuna (multiple genera), Haplochromis etc. of the Rift valley this can create instability in the social hierarchy of the group (Piefke et al., 2021). Of course removing some worst offending species would maybe be a benefit and therefore considering what species are to be included to start off. These voids in hierarchy can create more aggression as individuals are additionally are then identifying where they all fit. It’s a common assumption to assume aggression is male focused, as I discuss in a previous article females do frequently display aggression (Female Aggression).

Aggression can just be simply territorial, Neolamprologous cichlids can be good examples of this where individuals will maintain a space and the location based often on maintaining resources reproductive, predator protection and dietary. Fish sociality can be so very complex so it’s not entirely simple (Walter et al., 1994). This territoriality can also be seen in many Loricariids (Plecos) but also Anabantoids such as Channa or gourami’s, not always related to reproduction but just defense of a space.

Even many shoaling species might show aggression and therefore in that context a good number with an appropriate sex ratio would be best. Sociality in many species is important though, even if it is fleeting there is a behavioral enrichment, this isn’t always possible. But for shoaling and schooling species having others of the same species or population is important.

Many popular fishes can be counted as some what shoaling or schooling such as clown loaches, Chromobotia macracanthus to neon tetra, Paracheirodon innesi. These fishes alone or even in small numbers such as 3 or 6 depending on the species you might see abnormal behavior. Sometimes difficult to identify as some species are just illusive. A shoaling species kept alone could be illusive, swim erratically around the glass or even maybe shoaling with a different species. It’s definitely very context dependent. Sociality is important for behavioral and maybe even physiological development (Riley et al., 2018), we know that though from other verebtrates……. The understanding of species can be difficult but realistically multiple individuals of different species would not make up a shoal. Whether it be Corydoras or tetra, these species might not be closely related but even if they are it doesn’t mean they can communicate and therefore benefit socially. In other fishes such as discus communication is so much in their coloration given they can recognize individuals (Satoh et al., 2016), many varieties have lost or altered this ability and could result in communication issues between domestics, wilds and certain varieties. This means it’s highly likely discus can identify the different species and even populations. I have experienced this myself. Just by the fact so many fishes of the same genus or group are located together behavior likely results in this speciation.

Tankmates themselves can provide another kind of harm, they could be eaten and vice versa. Generally avoiding tankmates that will fit in the mouth is a good rule of measure. Some fishes can expand their mouth further then expected and while many are gape limited (limited by the size of their mouth) others this limitation is minor. Even the slowest fish are more then capable of eating faster fishes, discus feeding on cardinal/neon tetra is commonly accounted and goldfish frequently feed on smaller tankmates. The biggest risk here is if the smaller fish is too big to be swallowed down and that larger fish chokes.

The effect of the environment on fish disease.

More then often we think of the environment as purely about water parameters in fishes but the actual décor is important as well. I have been a long standing admin of goldfish groups and frequently seen where they have choked on gravel for example.

This is so diverse as so much can effect a fishes health.

Water parameters

The influence of water parameters on the health of fishes is so complex and so diverse, here I wont discuss hardness I have discussed it partially in the article on pH and hardness (here). Associated with mismanagement of hardness is gas bubble disease of which would require it’s own article or discussion, it is likely confused with a pH crash due to how quickly it occurs.

So, the main parameters we will focus on will be those related to nitrogen; ammonia, nitrites and nitrates. The values which cause death will vary depending on the fish and other parameters, many effects might seem asymptomatic unless a dissection is undertaken so do not assume because you cannot see an effect the fish isn’t affected. Ammonia itself burns the gills (Liu et al., 2021) but largely targets the the brain (Ip & Chew et al., 2010). Nitrite is the most well known as crossing into the blood combining with the blood to form methaemoglobin resulting in less oxygen carried around the body (Ciji & Akhtar, 2020), it can kill particularly rapidly seeming like a crash. These all are managed by a stable and cycled aquarium. Nitrate is maybe the least understood and the least studied as most studies focus on fishes with particular adaptability. This compound functions the same way as nitrites by combining with the blood to reduce oxygen saturation within the blood at higher volumes (Camargo et al., 2005). Many websites and a number of Youtubers focus on whether it kills but it shouldn’t be about rapid deaths rather long term effects, this is likely due to promoting methods where high nitrates are inevitable (Hrubec et al., 1996). So the question about low volumes becomes difficult, this is because most studies are short term and on fishes completely different from what we keep but even Hrubec et al. (1996) displays the toxicity of nitrate. It is better to assume toxicity then not just for the sake of water changes and half an hour or few hours a week, after all no one would argue for mammals to be left in their own waste because of intensive farming methods that do so.

Most of these nitrogen based compounds effect how oxygen is taken up whether it’s from burning the gills or the binding to the blood. So increasing oxygenation will not be harmful, methylene blue in small volumes is associated with increasing oxygen saturation in humans although it is difficult to find any research into the topic. Olufayo and Yusuf (2016) suggested that volumes of 67ppm to 199ppm of methylene blue didn’t increase oxygen saturation but previous research lists methylene blue as decreasing the solubility of oxygen (Khan et al., 2022). One aspect could be that largely when used in medicine it’s in humans and direct in the body, for fishes it’s added into the water so I think there is a whole other level of things at play. Another aspect of treating any higher values of these compounds is the use of Seachem prime, yes it does but decrease oxygen saturation so there is a balance (Seachem’s website).

Substrate

One of the things that least comes to mind when it comes to the health of fishes is substrates, there is so much diversity of them in stores and this can be overwhelming. Some might interact with the water parameters which is not usually ideal for many fishes as the focus of these is on plants.

The biggest risk substrate is gravels with any fish that might dig around in the substrate. It is frequently accounted particularly with goldfish where the gravel becomes stuck in either the oral or pharyngeal jaws of the fish.

Just as seriously from gravel is impaction although I have never encountered a certain case myself, if that gravel enters the digestive tract it wont exit easily unlike sand of which many fishes pass through their gut even in larger amounts harmlessly (Lujan et al., 2012).

Substrates offer an enrichment for so many fishes which is very important to recognise as so many species naturally search for food. I would rarely have a tank without it. The safest and most natural for many species is sands. One of these fishes that benefits so much are Corydoras and a number of loaches. Sharp substrates in these fishes is associated with erosion of the barbels present around the mouth although there is no studies on the topic and it is sometimes associated with the bacteria that gravel traps. This erosion leaves open wounds and an area highly open to infection.

décor

This is probably the most logical but in the stress of setting up a tank it can be a bit overwhelming to decide what to get. Generally consider if you are having any secretive fishes they will need plenty of spaces to hide in a variety of shapes and sizes, being exposed can be stressful for them and you’ll see the stress patterning on them. When it comes to these fishes always be sure there isn’t decor that the fish can get stuck on, this seems more of an issue with artificial décor then natural.

Sharp décor can also cause wounds whether it be the metal rods in silk plants or sharp dragon rock, for many fishes this is no issue but for clumsy species or some bottom dwellers accidents happen. Some species which might have very rough spawning or aggressive behaviour then they can bash against décor so maybe avoiding anything too rough here but we shouldn’t exchange enrichment to almost bubble wrap our fishes.

Some items might have a risk if eaten, I am not sure how true this is but some people do record rasping fishes such as Loricariids feeding on the paint of some artificial décor. This is probably best avoided just because of the potential chemicals. Similar any decor from plastics you are not familiar with and might degrade in the water releasing microplastics or other compounds.

Conclusion

Obviously for preventative care there is so much more that needs to be considered. Think about the habitats these fishes experience in the wild. Observe your fishes and their behaviour, sometimes it’s worth doing so from a distance so they aren’t expecting food either that or a camera to see them at night. Some behaviour is not always seen.

What are pathogens?

Pathogens are organisms that cause disease. These in fishes can be as followed:

• Viruses: Debatable if they are alive and therefore an organism. These cannot therefore do not have any treatment against them beyond keeping the immune system at it’s best and preventative care. Some viruses cannot be ‘cured’ and will always exist within the fish and some the fishes immune system will kill. Some viruses are fatal to the fish and others are not.
• Bacteria: This is a massive group of organisms. Unlike viruses and like all the other groups I will later list not all are pathogens or parasites, many have different roles. These can be killed by a variety of treatments and by the fishes immune system.
• Protozoa: This is a paraphyletic group of random organisms, not all are parasitic and some can be particularly difficult to treat while others much easier. A huge diversity of organisms.
• Annelid/worms: Very few are ever seen in the aquarium other then leeches, these are true worms and more tricky to treat.
• Nematodes: Similar to worms but not closely related at all. These are most often known for inhabiting the digestive tract or tissues in cysts. Many different wormers target this group.
• Copepods, isopods: Easier to see in most situations and best removed by hand if spotted, these larger Arthropod invertebrates can be difficult to treat once becoming an infestation. The only ones a fishkeeper will usually encounter is fish lice, Argulus sp.
• Algaes: The least obvious group to be parasitic/pathogenic but many are, velvet (Oodinium spp.) and related species with similar symptoms are reasonably common. This includes Cyanobacteria which are still algaes (Yanong et al., 2002). Technically as quite a lot of algae’s are protozoan and similar treatments sometimes work.

Disease and pathogen specificity.

Diseases and pathogens can be specific to certain groups of fishes or sometimes certain age demographics. A great example is some herpes type viruses many of which are specific to certain species or families (Hanson et al., 2011). This does mean if one fish has such a virus it wont be contracted by other taxa. You will also not find any confirmed cases of some diseases/pathogens in some fishes e.g. lymphocystis in carp and catfishes.

This is likely due to a diversity of different physiology between species, genera and even further families and more. Biology in general can differ so much certain groups might prevent access to certain taxa or they might lack the target organs and tissues.

Some individuals might be asymptomatic to certain diseases and pathogens.

Notes on Antibiotics

Antibiotic resistance is listed as one of the largest threats to humanity by the World Health Organisation, CDC etc. Bacteria and other pathogens are capable of resistance to a treatment on frequent exposure but bacteria being the most threatening. This has lead to many countries restricting their use, in countries like the UK they are only legal via prescription. Antibiotics unrestricted are commonly used without considering if the pathogen is actually a bacteria, if it is not it’ll have no effect but likely result in resistance of any bacteria around that are not currently a pathogen resulting in disease. In the aquarium there are many other treatments to try first. Antibiotics are reasonably specific to certain bacteria so would require knowing which are being targeted. If antibiotics are required then visit your vet, there are many fish vets within the UK who can be consulted on their opinion of the pathogen and course of treatment.

Pathogens of fishes and relevant articles to aid in diagnosis:

Spots, lumps and bumps: Click Here.

Sudden and rapid deaths (Unfinished)

Discolouration and change in skin/scale condition (Unfinished)

Abnormal external bodies (Unfinished)

References

Camargo, J. A., Alonso, A., & Salamanca, A. (2005). Nitrate toxicity to aquatic animals: a review with new data for freshwater invertebrates. Chemosphere, 58(9), 1255-1267.

Ciji, A., & Akhtar, M. S. (2020). Nitrite implications and its management strategies in aquaculture: A review. Reviews in Aquaculture, 12(2), 878-908.

Hanson, L., Dishon, A., & Kotler, M. (2011). Herpesviruses that infect fish. Viruses, 3(11), 2160-2191.

Hrubec, T. C., Smith, S. A., & Robertson, J. L. (1996). Nitrate toxicity: a potential problem of recirculating systems. Aquacultural Engineering Society Proceedings II: Successes and Failures in Commercial Recirculating Aquaculture. Northeast Regional Agricultural Engineering Service Cooperative Extension, Ithaca, NY.

Ip, Y. K., & Chew, S. F. (2010). Ammonia production, excretion, toxicity, and defense in fish: a review. Frontiers in physiology, 1, 134.

Khan, I., Saeed, K., Zekker, I., Zhang, B., Hendi, A. H., Ahmad, A., … & Khan, I. (2022). Review on methylene blue: Its properties, uses, toxicity and

Liu, M. J., Guo, H. Y., Liu, B., Zhu, K. C., Guo, L., Liu, B. S., … & Zhang, D. C. (2021). Gill oxidative damage caused by acute ammonia stress was reduced through the HIF-1α/NF-κb signaling pathway in golden pompano (Trachinotus ovatus). Ecotoxicology and Environmental Safety, 222, 112504.photodegradation. Water, 14(2), 242.

Lujan, N. K., Winemiller, K. O., & Armbruster, J. W. (2012). Trophic diversity in the evolution and community assembly of loricariid catfishes. BMC Evolutionary Biology, 12(1), 1-13.

Olufayo, M. O., & Yusuf, H. O. (2016). Toxicity of methylene blue on nile tilapia (Oreochromis Niloticus) juveniles. IOSR Journal of Environmental Science, Toxicology and Food Technology, 10, 9-16.

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Hybridisation is such a trigger word these days with many opinions for and against. First there are two definitions required:

Hybridisation: Interbreeding between two members of different species (Adah et al., 2013).

Crossbreeding: Interbreeding between two members of the same species, they might be the same variety/breed/cultivar or different.

These two terms are often used interchangeably even if they do not refer to the same event. Although these terms are different they do rely on the definition of species, genera and higher taxa which is hotly debated. One frequent definition being the biological species definition which relies on hybridisation being a barrier although is largely unreliable (Mallet, 2001).

Hybridisation as an event is not uncommon (Scribner et al., 2000), the event (or events) has resulted in the development of new species, a process known as speciation (Litsios & Salamin, 2014). Not all of the offspring might be fertile, many people in the aquarium trade might be familiar with the hybrid parrot cichlids (Amphilophus citrinellus x. Heros efasciatus), this may be the biggest barrier to speciation.

Barriers to Hybridisation

There are many barriers to hybridisation which really can effect the result if there is some compatibility.

Chromosomal number is the most well known barrier to hybridisation, chromosomes are units within the nucleus that contains all the DNA. Most humans have 22 pairs, fruit flies, Drosophila melanogaster have 4 pairs and the duckbilled platyplus has 26 (a lot of sex chromosomes though!), chromosomal number does not infer complexity at all and nor does the size of a genome. Generally different species might have a different number of chromosomes, this is very well displayed in the Loricariid (pleco) genus, Ancistrus (bristlenoses) which displays a diversity of different chromosomal numbers across a 50 or more species large grouping (Santos da Silva et al., 2022). During reproduction parental chromosome numbers ideally need to match to put simply. While this is a barrier it has resulted in infertile offspring, or in the example of the livebearers from Poecilidae a number of unusual adaptations. These hybrid Poecilidae have the ability to reproduce via gynogensis, asexual reproduction with the requirement of a male to stimulate the process, he contributes nothing to the offspring (Schultz, 1969) and this is displayed potentially in many species originating from hybridisation (Turner, 1980). The limitations of chromosomes really vary though but are definitely present.

Morphology is one of the obvious but so easily forgotten, organisms have to be morphologically compatible not just for the initial act but additionally for fertilisation and gestation. It might not seem it but that minor differences in morphology make a difference, what might seem obscure such as the shape of spermatozoa can differ resulting in the cell not being able to imbed into the egg, it’s even so diverse that the morphology of this cell can infer groups (Quagio-Grassiotto et al., 2020). Species often judge each other via their morphology and therefore there is some level of sameness particularly in the wild.

Behaviour, different species or even different populations might display very different behaviours. Behaviour itself is so integral to reproduction and mate choice, outside of captivity this is such a strong barrier.

Geography is maybe the most well known barrier and often can result in the other divisions. This can be within a small space such as fishes which inhabit deeper water whereas others the shallows. Barriers don’t always have to be spatial but they can also be temporal.

The issues with hybridisation

Hybridisation itself is not negative it’s just a force, in terms of conservation it can provide a variety of effects. Hybridisation can lead to the decline in a population of a species, most well known in the case of the Scottish wildcat (Felis silvestris silvestris) and the sister species, the domestic cat (Felis catus). As hybrids display what is known as a hybrid vigour (a mixture of traits between the two pariental species) it can change how these individuals interact with their environment and the genetic diversity of a species (Selz & Seehausen, 2019). The ecological impact of the introduction of hybrids can be described as unpredictable (Mandeville et al., 2022) which makes it difficult to identify those exact issues. Although, this argument is similar to introducing individuals who are selectively bred who might also influence the gene pool of wild individuals. It is generally not ideal, hybridisation in conservation is reserved for restoring species where no other alternative can be found e.g. Prezewalski’s horse (Equus ferus prezewalskii) and the domestic horse (Equus ferus caballus) in an effort to save the subspecies, although hybridisation is a risk to the subspecies (King, 2005).

But here we are talking about captivity, it’s highly unlikely any of our fishes will be involved in any real conservation effort or practices.

The main issue in captivity with these hybrids is so many do not go recorded, likely due to misidentifications of the parent species and lack of record keeping. Take Ancistrus dolichopterus, this species is generally diagnosed as having 8-9 soft dorsal rays, only one small individual was spotted excluding brown spots on the abdomen. While no seams were mentioned (Kner, 1854) it is commonly stated they display these, as with many other Ancistrus these often do fade with time and I have been shown individuals from the type locality lacking these seams. There are many spotted Ancistrus and over time localities and important information on them are lost. It’s more then often that these seams are how the judgement is based on what species it is. The point I am getting to is how well do we know these species and how they are defined? We are relying here on hybridisation not being able to occur but it likely does because species are kept together who are not the same species but assumed the same. I generally would assume unless wild caught for morphologically indistinctive Ancistrus, they should probably be assumed hybrids. I have frequently seen even Ancistrus ranunculus, a morphologically distinctive Ancistrus be misidentified so it’s a difficult situation. This doesn’t include how little we know about some genera, the frequency of bycatch and misidentifications from suppliers as well.

Surely those who argue against hybridisation should also argue against crossing those from different localities or populations. This is also changing the genetic landscape of a population and asks the question of how to define a species.

In captivity it does effect if you are getting what you think you are getting but for many this might not be an issue. One of the biggest worries though is because hybrids are a pick and mix of the parental species morphology and behaviour is largely unknown, it could be more similar to one parent then the other, an intermediate or totally different. Where parental species have different requirements this is also an issue.

How to avoid hybridisation

For wild caught fishes maybe getting fishes from the same source and checking locality matches if you are not completely confident on the identification. Keep records of who is being crossed with who.

It’s quite simple to keep those who could maybe hybridised apart, this is difficult to judge as it is not just about genera but if it looks similar don’t keep them together. If hybridisation occurs clearly record that and any offspring should be sold labelled as such, if possible to pet only homes with no worries of breeding where that label can be forgotten over generations.

For captive bred individuals the situation is difficult, if you aren’t sure and they aren’t from a responsible or reliable source then don’t buy unless you plan on just having the animals as a pet, assuming you don’t want to increase the number of hybrids.

Hybridisation Bias

Much of the dislike towards hybrids is focused on certain species although hybrids are much more common then they seem. Fishes such as domestic discus (Symphysodon spp.) are a mixture of the three different species and definitely the different localities depending on the variety (Ng et al., 2021). Some domestic species might have hybrid origins such as the common bristlenose, Ancistrus sp. but we do not know. These species do not seem to have health related defects from their morphology although those such as the parrot cichlid, Amphilophus sp. x. Vieja sp. (not to be confused with the true parrot cichlid, Hoplarchus psittacus), clearly does have health related deformities as a result of selective breeding and hybridisation as displayed in this link to a Monster FishKeepers discussion forum.

Conclusion

Hybridisation will always occur in the hobby, it is best that it is labelled so future keepers are aware of what their fishes are. If you are not completely sure of the species you are keeping then don’t breed it, ideally anyone who can identify the species should be able to cite a reliable website or the description of the species. Undescribed species might not have this so should have more precautions made such as keeping to one locality and morphology.

References

Adah, P. M., Onyia, L. U., & Obande, R. A. (2013). Fish hybridization in some catfishes: A review.

King, S. R. (2005). Extinct in the Wild to Endangered: the history of Przewalski’s horse (Equus ferus przewalskii) and its future conservation. Mongolian Journal of Biological Sciences, 3(2), 37-41.

Kner, R. (1854). Die Hypostomiden: Zweite Hauptgruppe der Familie der Panzerfische.(Loricata vel Goniodontes) (Vol. 1). KK Hof-und Staatsdruckerei.

Litsios, G., & Salamin, N. (2014). Hybridisation and diversification in the adaptive radiation of clownfishes. BMC Evolutionary Biology, 14, 1-9.

Mallet, J. (2001). Species, concepts of. Encyclopedia of biodiversity, 5, 427-440.

Mandeville, E. G., Hall Jr, R. O., & Buerkle, C. A. (2022). Ecological outcomes of hybridization vary extensively in Catostomus fishes. Evolution, 76(11), 2697-2711.

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.

Schultz, R. J. (1969). Hybridization, unisexuality, and polyploidy in the teleost Poeciliopsis (Poeciliidae) and other vertebrates. The American Naturalist, 103(934), 605-619.

Santos da Silva, K., Glugoski, L., Vicari, M. R., de Souza, A. C. P., Noronha, R. C. R., Pieczarka, J. C., & Nagamachi, C. Y. (2022). Chromosomal Diversification in Ancistrus Species (Siluriformes: Loricariidae) Inferred From Repetitive Sequence Analysis. Frontiers in Genetics, 13, 838462.

Scribner, K. T., Page, K. S., & Bartron, M. L. (2000). Hybridization in freshwater fishes: a review of case studies and cytonuclear methods of biological inference. Reviews in Fish Biology and Fisheries, 10, 293-323.

Selz, O. M., & Seehausen, O. (2019). Interspecific hybridization can generate functional novelty in cichlid fish. Proceedings of the Royal Society B, 286(1913), 20191621.

Turner, B. J., Brett, B. L. H., & Miller, R. R. (1980). Interspecific hybridization and the evolutionary origin of a gynogenetic fish, Poecilia formosa. Evolution, 34(5), 917-922.

Quagio-Grassiotto, I., Baicere-Silva, C. M., de Oliveira Santana, J. C., & Mirande, J. M. (2020). Spermiogenesis and sperm ultrastructure as sources of phylogenetic characters. The example of characid fishes (Teleostei: Characiformes). Zoologischer Anzeiger, 289, 77-86.