The API Nitrite Testing Kit is a liquid test kit that was designed to measure nitrite levels in both fresh and saltwater aquariums. Toxic nitrite is produced by nitrifying bacteria in the biological filter as it breaks down ammonia. Nitrite in aquarium water is harmful and life-threatening to the fish and other living organisms within the tank. The API Nitrite Testing kit tests for harmful nitrite levels measuring from 0 to 5.0 parts per million. The kit includes one bottle which equals 180 tests.