Abstract: Ammonia distributes widely in aquaculture water, and is a major issue in the massive mortality rate of fish species with a high ammonia aquaculture environment. Studies on ammonia toxicity and defense in fish are important because of ecological, environmental, and economical relevance. Some fish species have specific strategies to deal with ammonia loading, so that they can tolerate high levels of environmental or internal ammonia. These strategies can be categorized into: (1) glutamine synthesis; (2) urea synthesis and excretion; (3) active \rmNH_4^ + excretion; (4) ammonia detoxification, improved by Rh glycoproteins; (5) lowering of ambient pH; (6) NH₃ volatilization and alkalization of the body surface; (7) reduction in body ammonia production; (8) amino acid catabolism leading to the alanine form; and (9) high tissue and organ ammonia tolerance. The response of fish species that are able to ameliorate ammonia toxicity are many and varied, depending on the behaviour of the species and its habitat environment. This paper summarizes ammonia toxicity, as it is hoped that this review can provide basic information on ammonia detoxification mechanisms in air-breathing fish species.