Brown blood disease

Nitrite (NO2-) is toxic for fish when it is absorbed by fish and reacts with hemoglobin to form methemoglobin (Met-Hb). In this reaction, the iron in the “hem” of hemoglobin is oxidized from ferrous to ferric state, so it can not combine with oxygen to bring this compound to the essential organs of the fish. For that reason, reducing activity of hemoglobin or anemia iscalled nitrite toxicity or metheglobinemia. The blood of fish that contains significant amounts of methehemoglobin is brown in colour, so the common term for this poison is “brown blood disease”