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Nitrite is produced in the aquarium by the biological filter. Beneficial bacteria in the biological filter convert toxic ammonia into nitrite (also toxic). The biological filter then converts nitrite into nitrate. Testing for the presence of toxic nitrite is essential, so that once detected, steps can be taken to remove it. Otherwise, nitrite in the aquarium will prevent fish from carrying on normal respiration. High levels of nitrite quickly lead to fish death. Even trace amounts of nitrite stress fish, suppressing their immune system and, thereby, increase the likelihood of disease and subsequent fish loss.