Fish produce ammonia from their gills and in their waste.
Ammonia also is also released through the decomposition of food and other organic compounds in the aquarium. Ammonia is toxic to fish. In sufficient concentrations, it can kill fish rapidly and in lower concentrations it will kill them if the condition is prolonged. Even if the ammonia doesn t kill directly by burning their gills, it will weaken the fish and they will succumb to opportunistic disease organisms that are constantly present in any aquarium. When the pH of the water is in the acid range, much of the ammonia will be present as ammonium which is not toxic.