Nitrite toxicity is primarily a problem of freshwater systems. Inadequate biological filtration, overfeeding, overcrowding, an insult to nitrifying bacteria, or a dramatic water change may cause this condition. The incidence of death and illness is usually high. Fish may struggle to breathe and their gills may be light brown or tan in color. The color of freshly drawn blood may be light brown. In many cases, other gross lesions (problems visible to the naked eye) are not evident.
Nitrite is an intermediate compound in the nitrogen cycle and is converted to nitrate by the Nitrobacter bacteria of a healthy biological filter. In a freshwater tank, levels above 0.05 parts per million (ppm) will likely be harmful to the fish. The problem is common during the "new tank syndrome" after the ammonia has been converted to nitrite by Nitrosomonas bacteria. This usually occurs between weeks one and two of a cycling biological filter.
Nitrite enters the fish by crossing the gill membrane. Problems arise because nitrite is capable of oxidizing (chemically changing) hemoglobin to methemoglobin in the bloodstream. This condition is known as methemoglobinemia and can be fatal since methemoglobin is an inefficient carrier of oxygen.
Brackish and marine fish are not particularly susceptible to nitrite toxicity since chloride salts compete with nitrite for uptake at the gill lamellar surface. In fact, adding salt to the aquarium is a standard treatment for nitrite toxicity.
In diagnosing nitrite toxicity, your veterinarian will rule out other water quality related problems including heavy metal toxicity. Once treatment protocols have been instituted, the prognosis for recovery is usually good as long as other water quality problems and secondary infections do not exist.
Nearly all standard water test kits measure dissolved nitrite. While the cause of increased nitrite levels needs to be determined, immediate intervention is necessary, especially when nitrite levels are above 0.5 ppm.
Chloride should be added to the affected water system as soon as possible. In most cases, the addition of 1.0 parts per thousand (ppt) salt (sodium chloride) will yield approximately 500 ppm chloride (more than enough to do the job). One part per thousand is the same as 1 gram per liter (about 4 grams per gallon or roughly 1 pound per 100 gallons). Most species of ornamental fish will readily tolerate this much salt.
Prevention is the best solution for nitrite poisoning. By not overstocking, overfeeding, or overmedicating your aquarium, the chances of a nitrite problem are greatly reduced. Coupling these strategies with an adequately functioning biological filter will insure that nitrite toxicity will be something you only read about.