Ammonia is a byproduct of fish metabolism and the decomposition of organic matter. It exists in water in two forms, ionized and un-ionized. Un-ionized ammonia concentration is directly related to pH levels in water. Higher pH levels raise NH3-N concentration in water, increasing its level of toxicity to fish. Un-ionized ammonia toxicity causes osmoregulation imbalance, kidney failure, and gill damage.
Nitrification occurs in two steps.
In the first step, ionized ammonia is converted to nitrite by the action of the Nitrosomonas bacteria. Therefore, nitrite presence in water is usually accompanied by low levels of dissolved oxygen. Both bicarbonate and carbonate compounds are main contributors to water alkalinity. Water with low hardness may have a reduced buffering capacity despite the presence of high total alkalinity. Thus fish species for marine aquaculture differ from those of fresh or brackish waters.