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.

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.

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).

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.