Nitrosomonas bacteria oxidise ammonia and form the ion nitrite (NO2-). While not as toxic as ammonia, nitrite is still undesirable in an aquarium.
Nitrite reacts with haemoglobin in red blood cells forming methaemoglobin which effectively blocks the oxygen uptake to the fish’s bloodstream.
The lack of oxygen causes the fish to become lethargic and stop eating eventually resulting in death. Methaemoglobin also turns the blood brown and this is most noticeable in the gills. Many fish keepers may refer to this as “brown blood disease”.
Like ammonia, the ideal level of nitrite in an aquarium is 0 mg/l or zero parts per million (ppm).
Susceptibility depends very much on the fish; some will tolerate levels of up to 100 mg/l. However, levels as low as 2 mg/l will make fish more prone to disease attack and most fish will not survive at this level for more than 4 days.
Arguably (as you will encounter high nitrites during cycling or possibly after filter maintenance), any reading above 2 mg/l indicates a water quality problem that must be corrected.
Although I mentioned earlier that some fish would tolerate higher nitrite levels, keeping nitrite to a minimum will ensure that you do not encounter too many problems. Check nitrite periodically using an appropriate test kit.