There is much debate within the hobby as to the maximum level of nitrate allowable in our tanks. Recommended numbers fluctuate anywhere from 100ppm to 400ppm, with 50ppm - 100ppm 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.
Numerous articles & books 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?
The problem with identifying a baseline level of recommended nitrate is it really depends 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, University of Florida on Gasmbusia in Florida Springs (3):
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.
These studies identify 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. "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 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).
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? One study conducted by the Virginia-Maryland Regional College of Veterinary Medicine on Hybrid Striped Bass detailed 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 the fish became blind and they began dieing seven weeks into the experiment. Autopsies revealed elevated nitrate concentrations resulted in the following physiological impacts:
Affects antibody production
Increased number of immature red blood cells
Lowered level of mature red blood cells (anemia)
Higher count of monocyte (a specific white blood cell)
Higher count of neutrophil (a specific white blood cell that is especially destructive to microorganisms)
Higher count of TLC - Thrombocyte-like cell (a blood cell of nonmammalian vertebrates that promotes blood clotting)
Higher levels of creatine (A nitrogenous organic acid found in muscle tissue that supplies energy for muscle contraction)
Higher calcium values in the blood
Lower Chloride values in the blood
Autopsy revealed damage to the spleen, liver, and kidneys
Other conclusions reached:
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).
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.
What Level of Nitrates Should Be Allowed
Most of the scientific studies 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 or what levels impact reproduction. It is not surprising that the NASA test, specifically conducted to determine maximum long-term levels of nitrate, 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).
Some are of the opinion that nitrate is perfectly safe at 80ppm or 100ppm. For some fish, this may be true. But I do not agree. Impacts may not be visible to the aquarist / aquaponist, but I believe a blood test would reveal a difference.
Nitrate, like ammonia, is a toxin. Fish bred in captivity far longer than our food fish (goldfish are a good example) are no more tolerant to ammonia & nitrate than are their wild counterparts. Adaptation to a toxin is an evolutionary step, requiring mutation, not something an organism can simply become accustomed to.
Of all of the articles, 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.
(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)