Fish urine contains ammonia, urea, and other nitrogenous waste products. Ammonia is the primary waste product produced by fish, and it is toxic to fish at high concentrations. Urea is a less toxic form of nitrogenous waste that is produced when the fish's body breaks down proteins. Fish urine also contains salts, electrolytes, and other dissolved substances that are necessary for maintaining proper chemical balance in the fish's body and in the water surrounding the fish.