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Nitrites are a toxic waste material found in varying concentrations in most aquariums. It is produced by nitrifying bacteria in the biological filter as it breaks down ammonia. As the filter develops and grows it uses the nitrites as a food source and breaks them down into a less harmful nitrate. Regular testing should be done because low levels of nitrites can cause problems in the tank causing your fish to have reduced oxygen which could lead to suffocation and death.
The chart shows levels from 0-3.
The above chart was used from a pamphlet received in the Aquarium Pharmaceuticals, INC Nitrite testing kit.