Nitrite (NO₂^-) is formed from the oxidation of ammonium (NH₄⁺) in the aquatic environment. Nitrifying bacteria, Nitrosomonas spp., oxidise ammonium into nitrite. The bacteria Nitrobacter spp. then convert nitrite into nitrate (NO₃; Lewis & Morris, 1986). Nitrite can be found in high concentrations naturally, such as in deep stratified lakes in the hypolimnetic layer (Boyd 1990). Within aquaria, the primary source of nitrite is the oxidation of ammonium produced by the fish. Nitrite concentrations may increase if oxidation rates of ammonia exceed oxidation rates of nitrite (Colt & Tomasso, 2001), or if the oxidation process is inhibited (Russo & Thurston, 1991). A secondary source of nitrite is anaerobic denitrification, converting nitrate in nitrite (Boyd, 1995).

Nitrite may be present in water as nitrite ion (NO₂^-) or nitrous acid (HNO₂). As with the NH₄⁺/NH₃ equilibrium, the balance between NO₂^- /HNO₂ is pH dependent. The higher the pH the more the equilibrium will shify in the direction of NO₂^- (see graph) .

Toxicity

Nitrite is toxic to fish as it diffuses from the blood plasma into the red blood cells, where it oxidises the Fe²⁺ in haemoglobin (Hb) to the Fe³⁺ oxidation state, converting haemoglobin into methaemoglobin (metHb). MetHb lacks the capacity to bind to oxygen, resulting in hypoxia. The build up of MetHb is commonly known as brown blood disease, named after the characteristic colour of blood and gills of chronically nitrite-exposed fish or other animals. MetHb occurs naturally in the blood of fish. Levels in excess of 10% are detrimental to fish health, and clinical signs have been reported with levels over 25% (Lewis & Morris 1986). Nitrite exposure may also damage the gills (hypertrophy, hyperplasia, epithelial separation) and the thymus (haemorrhage and necrotic lesions; Wedemeyer, 1996). Luckily, nitrite-induced metHb is a reversible condition (Scott & Harrigan & 1985). If nitrite levels in the water are reduced before metHb levels become lethal, the fish should fully recover (Jensen 2003).
Aside from the indicative brown blood found in exposed fish, gross signs of methaemoglobinaemia (a.k.a. brown blood disease) are lethargy as blood levels of metHb approach 70-80%, with disorientation and unresponsiveness reported at levels near 100% (Westin 1974). The lethargy and lack of activity reported in fish with methaemoglobinaemia may well be a behavioural response to cope with the condition, as this reduces their oxygen demand. However, should the fish be startled or forced to become active, they may then die from hypoxia (Huey et al. 1980).

Recommended levels

Recommended maximum levels vary quite a bit, i.e. between 0.01 to 0.2 mg/l. One reason for this is that the toxicity of nitrite depends strongly on the pH (toxicity of nitrite increases at decreasing pH) en the concentration of Cl^- (toxicity of nitrite decreases at increasing Cl^- concentration). In general, if your aquarium has a properly functioning filter, you should not be able to detect nitrite.

How to avoid high nitrite levels

Several methods can be applied to avoid exposing the fish to high concentrations of ammonia:

1. Do not directly introduce fish to a new aquarium. The bacterial community in the filter needs time to develop. Also be aware that cleaning the filter or medicine use may severely affect the efficiency of a filter.
2. Don’t stock more fish than the filter is capable of dealing with.
3. Don’t overfeed your fish.
4. Use a sufficient amount of plants in your tank.

References

Boyd, C.E., 1990. Water Quality in Ponds for Aquaculture. Auburn University, Alabama.
Boyd, C.E., 1995. Bottom soils, sediment, and pond aqauculture. Chapman & Hall, New York, pp. 348.
Colt, J.E., Tomasso, J.R., 2001. Hatchery water supply and treatment. In: G.A. Wedemeyer (Ed.) Fish Hatchery Management (2nd edition), pp. 91-186. American Fisheries Society, Maryland.
Eddy, F.B., Kunzlik, P.A., Bath, R.N., 1983. Uptake and loss of nitrite from the blood of rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L., in fresh water and in dilute sea water. Journal of Fish Biology, 23,105-116.
Huey, D.W., Simco, B.A., Criswell, D.W., 1980. Nitrite-induced methemoglobin formation in channel catfish. Transactions of the American Fisheries Society, 109, 558-62.
Jensen, E.B., 2003. Nitrite disrupts multiple physiological functions in aquatic animals. Comparative and Physiological Biochemistry A, 135, 9-24.
Lewis, W.M. Jr., Morris, D.P., 1986. Toxicity of nitrite to fish: a review. Transactions of the American Fisheries Society, 115, 183-95.
Russo, R.C., Thurston, R.V., 1991. Toxicity of ammonia, nitrite, and nitrate to fishes. In: D.E. Brune & J.R. Tomasso (Eds) Aquaculture and Water Quality, pp. 58-89. World Aquaculture Society, Baton Rouge.
Scott, E.M. & Harrington, J.P., 1985. Methemoglobin reductase activity in fish erythrocytes. Comparative Biochemistry and Physiology B, 82, 511-513.
Wedemeyer, G.A., 1996. Physiology of Fish in Intensive Culture Systems. Chapman & Hall, London.
Westin, D.T., 1974. Nitrate and nitrite toxicity to salmonid fishes. Progressive Fish Culturist, 36, 86-9.