Most fish and freshwater invertebrates excrete ammonia as their principle nitrogenous waste. The proportion of TAN that exists in ionized and un-ionized form varies with pH and temperature. Fish continuously exposed to more than 0.02 ppm of the un-ionized form may exhibit reduced growth and increased susceptibility to disease. When fish are cultured intensively and fed protein-rich feeds they can produce high concentrations of ammonia in the water. Ammonia and other metabolic wastes are gradually removed by natural processes in ponds or through the use of biological filters in recirculating and reuse systems.
Ammonia is removed by bacteria that initially convert it into nitrite and subsequently into nitrate.
Concentrations of 0.5 ppm have reduced growth and adversely affected fish. Fish can tolerate nitrate to several hundred ppm. Fish survive and grow best in waters with a pH between 6-9. If pH readings are outside this range, fish growth is reduced. At values below 4.5 or above 10, mortalities occur.