API Nitrite TestToxic Nitrite is produced by nitrifying bacteria in the biological filter as it breaks down ammonia. Reads nitrite levels from 0 to 5ppm. For fresh & saltwater useNitrite, a major cause of fish death, is produced in the aquarium by fish waste and overfeeding. Even low levels affect the red blood cells of fish reducing their ability to carry oxygen, thus causing suffocation and even death.