Fish do urinate. It may be hard to tell that fish pee, because they live underwater, but if a tank starts to smell like ammonia after not being cleaned for a while, this is actually because fish do urinate.
Saltwater fish urinate through their gills because of all the salt they take in; however, they urinate less than freshwater fish. Freshwater fish urinate through their urinary pores as a method to excrete the mass amount of water their bodies take in. In addition to ammonia, fish urine also contains amino acids, organic acids called creatine and creatinine and some urea.