Is Nitrate Toxic? A Study of Nitrate Toxicity

Written by Kmuda. Posted in All About the Water

There is much debate within the hobby as to the maximum level of nitrate allowable in our tanks. Recommended numbers fluctuate anywhere from 20ppm to 100ppm, with 40ppm - 50ppm being generally accepted (not by me), and 20ppm being suggested for Nitrate sensitive fish. I personally, adhere to the less than 20ppm philosophy for any fish we keep in our aquariums.

Numerous articles, books, and other forms of information have historically label nitrates as “harmless”. This has absolutely proven to be false. Nitrates are indeed toxic. The question is at what concentration does toxicity begin?

Different Tolerances for Different Species

The problem with identifying a baseline level of recommended nitrate is it really depends specifically on the species of fish. Some fish are designed by nature to withstand elevated nitrate levels and others will be impacted at very low levels, as evidenced by the below excerpt from a study conducted by the University of Florida on Gasmbusia in Florida Springs (3):

Note: mg/L can be directly translated into ppm. They are the same measurement


  • Evidence suggests that sensitivity to nitrate is species-specific. Kincheloe et al. (1979) reported larval mortality of Chinook salmon, rainbow trout, and cutthroat trout at concentrations as low as 2.3-7.6 mg/L NO3-N. The 96-hr LC50 (median lethal concentration) for fathead minnow larvae is 1,341 mg/L NO3-N (Scott and Crunkilton 2000), and the lethal dose for adult and juvenile medaka is 100 mg/L NO3-N (Shimura et al. 2002).
  • A range of sublethal effects of nitrate has also been reported. For example, Greenlee et al. (2004) observed increased apoptosis and reduced cell number in cultured preimplantation mouse embryos exposed to 1 mg/L ammonium nitrate. In an accumulated nitrate test, in which nitrate built up over the course the experiment, Shimura et al. (2002) observed delayed hatching time and reduced fertilization and hatching rates of eggs produced by adult medaka exposed for 2 months to a maximum of 75 mg/L NO3-N. In that test, the offspring also exhibited reduced juvenile growth rates. At 50 mg/L NO3-N, Shimura et al. (2002) observed reduced spawning and fecundity (measured as egg number) among adult medaka exposed to nitrate as juveniles.

So, what does all of this scientific mumbo jumbo technospeak tell us. It identifies that amongst three different species of fish, exposed to nitrates for 96 hours, one species of fish could only withstand 2.3-7.6ppm before death, another (the Fathead Minnow) 1,341ppm, and another 100ppm. These levels of nitrates can kill quickly (like ammonia), so we at least have confirmation that dispels the myth that nitrates are not toxic. The statement concerning the spawning of Meduka fish also suggests that short term exposure to less than lethal levels of nitrates results in physiological impacts as does the most important statement above.... "A range of sublethal effects of nitrate has also been reported".

So we know this, at least when it comes to short term exposure to very high levels of nitrate, there is a wide ranging concentration of what would be considered lethal and we know that even if not at “quick kill” levels, nitrates have a physiological impact.

Long Term vs. Short Term

I believe a lot of the confusion associated with suggesting maximum nitrate levels is that there is a vast difference between levels of nitrate that are determined to be lethal (median lethal concentration – MDL or LC50) and levels of nitrate determined to be safe for long term exposure. As an example, the MDL for adult medeka fish is 96 hours at 100ppm while the maximum level considered safe for long-term exposure is less than 25ppm (7). Literature and web sites often quote statistics associated with the median lethal concentration without taking into consideration that this is the point at which fish are going to die directly from nitrate poisoning, not the concentration at which nitrates begin to have sublethal physiological effects on fish under long term exposure. I should also clarify that studies reveal that the MDL decreases as the exposure time increases.

Physiological Effects of Nitrates

Some detailed studies on the affects of nitrates are listed below:

  • Research conducted for NASA designed to identify requirements for the long term raising of Medaka fish in space recommends keeping nitrates below 25ppm.
  • Studies performed on Gambusia in Florida springs discovered that decreased fertility rates were caused by nitrate in concentrations as low as 1.5ppm.
  • A recent study which reviewed all prior studies on the impacts of nitrates suggests that the most sensitive freshwater invertebrates and fish are affected by nitrate concentration as low as 2ppm, with the primary physiological impact being a decreased ability of the blood to carry oxygen (anemia).

So what physiological impacts do high levels of nitrate have on fish? Unfortunately, available information is limited. One study conducted by the Virginia-Maryland Regional College of Veterinary Medicine on Hybrid Striped Bass is the best I’ve located at detailing the physiological impacts of elevated nitrate.

This test identified that nitrates at 200ppm can kill relatively quickly. Within one week of being exposed to nitrates at this level (three weeks into the test) the fish became blind and they began dieing seven weeks into the experiment. Autopsies revealed elevated nitrate concentrations resulted in the following physiological impacts:

  1. Affects antibody production
  2. Increased number of immature red blood cells
  3. Lowered level of mature red blood cells (anemia)
  4. Higher count of monocyte (a specific white blood cell)
  5. Higher count of neutrophil (a specific white blood cell that is especially destructive to microorganisms)
  6. Higher count of TLC - Thrombocyte-like cell (a blood cell of nonmammalian vertebrates that promotes blood clotting)
  7. Higher levels of creatine (A nitrogenous organic acid found in muscle tissue that supplies energy for muscle contraction)
  8. Higher calcium values in the blood
  9. Lower Chloride values in the blood
  10. Autopsy revealed damage to the spleen, liver, and kidneys

Other conclusions reached:

  1. Nitrate damages the gills and kidneys affecting osmoregulatory ability (ability of the fish to regulate fluid levels and release toxins, something we do via urination, something they do via osmoregulation).
  2. The observed changes are the result of a pathological response and not of a generalized stress response.

So what does the abnormal blood chemistry indicate? In short, it means the fish are suffering from infection, severe physical stress, and tissue damage. Their blood is incapable of distributing sufficient oxygen, the immune system is in overdrive and has become deficient, and the kidneys are failing.

Many will step back and say that these results are only because the Nitrates were severely excessive. This may be true, but the question is at what point do these impacts begin? Is it possible that these physiological changes can be detected in fish subjected to much lower concentration of nitrates over a longer period of time? Unfortunately, I have not found a direct answer to this question; as research on the subject is lacking, but there is evidence that sublethal nitrate toxicity actually begins at lower concentrations, with the physiological impacts increasing as either the concentration of nitrates increase or the duration of exposure increases.

At Last, What Level of Nitrates Should Be Allowed

As mentioned, most of the scientific studies I’ve found do not take into consideration long-term life of the fish. They are designed to determine what level becomes lethal within a short amount of time (generally 36-96 hours) or what levels impact reproduction. I don't find it surprising that the only test I am aware of (the NASA test), specifically conducted to determine maximum long-term levels of nitrate (for the meduka fish), recommends keeping nitrates below 25ppm (7). A study on Fathead minnows attempting to determine when nitrate induced physiological impacts could be detected returned a result of 21ppm, with significant physiological differences being detected at 41.6ppm (6). A compiled review of prior testing conducted in Spain at the Universidad de Alcala suggests that the effects of nitrate toxicity in the most sensitive freshwater species can begin in concentrations as low as 2ppm and that long term exposure to nitrates in concentrations of 10ppm are known to adversely effect rainbow trout, chinook salmon, and cutthroat trout (1) (5). Only studies of channel catfish returned a result of what I would consider a high acceptable range, and that was at 90ppm (2).

Now I want to step back here…. remember the quotes from the beginning of the article, where it identifies a 96 hour MDL of 1,341ppm (Wow, that's a lot) for Fathead minnows. That means within 96 hours of being exposed to that level of nitrates the fish died, but it is also 100 to 1,000 times higher than any other fish in this particular study. However, this is the exact same species just mentioned in the above study, where testing identified the physiological effects of nitrates could be detected in concentrations as little as 21ppm (on the exact same species - Fathead minnow) and these impacts became “significant” at 41.6ppm. Think about that. Fathead Minnows are an extremely hardy fish, considered amongst the most hardy of all fish, capable of withstanding levels of ammonia and nitrate far beyond most other fish, and the physiological impacts of nitrates can be detected in this species at concentrations as low as 21ppm. Do we think our Oscars and South American Cichlids can do better? When Oscars and South American Cichlid's have evolved to survive in near nitrate free water and Fathead Minnows are commonly found in polluted waters incapable of supporting other species of fish?

I use an analogy comparing our fish’s long-term exposure to nitrates to humans smoking. The effects of smoking differ per individual. Some will experience smoking related difficulties and disease within a shorter amount of time than others. Some will experience these issues even though they smoke less than others who have not yet experienced smoking related problems. But in general, those who smoke 2 packs per day for 20 years will experience more devastating illness than those that smoke ½ pack per day for 20 years. In addition, smoking 10 packs a day for 4 weeks is vastly different than smoking ½ a pack per day for 10 years. Our problem in determining suggested Nitrate levels, to further continue the analogy, is that most Nitrate related testing is conducted to determine the impacts of smoking 10 packs a day for 4 weeks as opposed to the impacts of smoking ½ pack a day for 10 years.

So, what do we define as the suggested maximum level of allowable nitrate in our tanks? Historically, I have attempted to keep nitrates below 20ppm and strive, in my Oscar tank, to keep them below 10ppm (for optimal growth and coloration). I think this is validated by the studies researched (and referenced) during the creation of this article and I can assure you, the vast majority of fish we keep will be healthy and thrive if maintained with nitrates at either of these levels. My goal, to continue with the smoking analogy, is the make my fish smoke free.

Wild Oscars are only subjected to minute traces of nitrate. Long-term studies identified a nitrate concentration high of 1.24ppm in the Amazon River (9) and 80ppb (virtually undetectable) in the Orinoco River (10). Although some Oscars stranded by receding waters in the dry season may face higher concentrations of nitrate, this is only for short periods of time (a few months at most), and the effects this has on those fish that survive these pools are completely unknown.


Some are of the opinion that nitrate is perfectly safe at 40ppm or 50ppm. For some fish, this may be true. But I do not agree. Impacts may not be visible to the aquarist, but I believe a blood test would reveal a difference. Are these impacts life threatening? Not in the short term, perhaps not even in the long term, but I feel safe in stating that a fish kept in nitrate concentrations of 40ppm will not grow as large or live as long as a fish kept at 20ppm, nor one kept at 20ppm as one kept at 10ppm. In addition, I find it difficult to believe that fish, such as Oscars, that face such low levels of nitrate in their native habitat, after only less than a century of commercial captive breeding, have developed a capacity to withstand long term concentrations of nitrate in excess of 3000% higher than their wild counterparts. Nitrate, like ammonia, is a toxin. Fish bred in captivity far longer than Oscars (goldfish are a good example) are no more tolerant to ammonia than are their wild counterparts. Adaptation to a toxin is an evolutionary step, requiring mutation, not something an organism can simply become accustomed to, and (if anything), the inbreeding of ornamental fish has resulted in a weaker genetic base, not a stronger one.

Of all of the articles I researched, only one (pertaining to Channel Catfish (2)) identified anything above 25ppm as an acceptable safe level of nitrate for long term exposure and this study was geared more towards discerning short term effects, not long term. There are numerous statements on web sites to the contrary, but I restricted data research to scientifically reviewed articles.

For a somewhat opposing view on this subject, and associated discussion, review this article.


  • (1) Camargo, Julio A., 2004, Nitrate Toxicity to aquatic animals: a Review with new data for freshwater invertebrates, Chemosphere 58 (2005) 1255-1267
  • (2) Colt, J., Tchobanoglous, G., 1976. Evaluation of the short-term toxicity of nitrogenous compounds to channel catfish, Ictalurus punctatus. Aquaculture 8, 209–221
  • (3) Edwards. Thea M., 2006, Water Quality Influences Reproduction In Female Mosquitofish (Gambusia holbrookie) from Eight Florida Springs, Environment Health Perspectives, Vol 114, Supplement 1
  • (4) Hrubec, Terry C., Nitrate Toxicity: A Problem of Recirculating System, Aquatic Medicine Laboratiry, Virginia-Maryland Regional College Of Veterinary Medicine.
  • (5) Kincheloe, J.W., Wedemeyer, G.A., Koch, D.L., 1979. Toler- ance of developing salmonid eggs and fry to nitrate exposure. Bull. Environ. Contam. Toxicol. 23, 575–578
  • (6) Scott, G., Crunkilton, R.L., 2000. Acute and chronic toxicity of nitrate to fathead minnows (Pimephales promelas), Cerio- daphnia dubia and Daphnia magna. Environ. Toxicol. Chem. 19, 2918–2922
  • (7) Shimura, Ryuji, 2004, Nitrate Toxicity on Visceral Organs of Medaka Fish, Oryzias latipes : Aiming to Raise Fish From Egg To Egg in Space, Biological Sciences in Space, Vol. 18 No1 (2004):7-12
  • (8) DELLAMANO-OLIVEIRA, Maria José, SENNA, Pedro Américo Cabral and TANIGUCHI, Glória Massae. Limnological characteristics and seasonal changes in density and diversity of the phytoplanktonic community at the Caçó pond, Maranhão State, Brazil. Braz. arch. biol. technol. [online]. 2003, vol. 46, no. 4 [cited 2006-11-03], pp. 641-651.
  • (9) Mayorga, Emilio, 2002, Processing of Bioactive Elements in the Amazon River System, The Ecohydrology of South American Rivers and Wetlands. IAHS Special Publication no. 6, 2002
  • (10) Lewis, William M., 2006, Concentration and transport of dissolved and suspended substances in the Orinoco River, Biodegradation Volume 7, Number 3 / May, 1989, 0923-9820 (Print) 1572-9729 (Online)