Ammonia is toxic to fish if allowed to accumulate in fish production systems. When ammonia accumulates to toxic levels, fish cannot extract energy from feed efficiently. If the ammonia concentration gets high enough, the fish will become lethargic and eventually fall into a coma and die.
In properly managed fish ponds, ammonia seldom accumulates to lethal concentrations. However, ammonia can have so-called “sublethal” effects—such as reduced growth, poor feed conversion, and reduced disease resistance—at concentrations that are lower than lethal concentrations
Via John Bostock