Nitrite enters a fish culture system after feed is digested by fish and the excess nitrogen is converted into ammonia, which is then excreted as waste into the water.
Total ammonia nitrogen (TAN; NH3 and NH4+) is then converted to nitrite (NO2) which, under normal conditions, is quickly converted to non-toxic nitrate (NO3) by naturally occurring bacteria. Uneaten (wasted) feed and other organic material also break down into ammonia, nitrite, and nitrate in a similar manner. Brown blood disease occurs in fish when water contains high nitrite concentrations. Nitrite enters the bloodstream through the gills and turns the blood to a chocolate-brown color. Haemoglobin, which transports oxygen in the blood, combines with nitrite to form methaemoglobin, which is incapable of oxygen transport. Brown blood cannot carry enough oxygen, and affected fish can suffocate despite adequate oxygen concentration in the water. This accounts for the gasping behaviour often observed in fish with brown blood disease, even when oxygen levels are relatively high. Nitrite (NO₂) – nitrifying bacteria living in the filter and aquarium convert ammonia to nitrite (NO₂). A rise in nitrite usually follows an ammonia spike. Nitrite inhibits oxygen exchange to fishes’ bloodstream, thereby causing them to suffocate. Fish that are experiencing nitrite toxicity will often breathe rapidly and gasp at the surface (although there are other reasons they do this), and their gill filaments will turn from bright red to dull brown or grey in color. The only safe nitrite level is zero.