If you are a fish farmer, you know that they defecate, but it's harder to discern whether they urinate. However, if you don't clean the tank regularly and start smelling
ammonia, there's your answer.
Fish do urinate, but freshwater and saltwater fish go about it in a different fashion. According to the Virginia-Maryland Regional College of Veterinary Medicine website, fish basically urinate all the time.
How Fish Urinate
Fish urinate either through their gills or through a "urinary pore." The latter eliminates urine that has been filtered via the kidneys. Saltwater varieties excrete most urine through the gills, while freshwater fishes do so through the urinary pore. Saltwater fish take in a great deal of salt and must excrete it. Freshwater fish have to get rid of all the water they take in.
Fish have two kidneys.
The head kidney filters out wastes that go through the gills, while the posterior kidney filters wastes that go out the urinary pore.
In freshwater fish, the kidneys save ions and excrete water.
For saltwater fish, the kidneys conserve water and eliminate ions, with most of this waste material exiting through the gills.
While fish urine primarily consists of ammonia, that's not all it contains.
Depending on the species, ammonia leaves the body primarily via the gills.
Fish urine also includes amino acids, a small amount of urea and the organic acids creatinine and creatine.
Saltwater fish urine contains a great amount of salt, while freshwater fish urine consists primarily of water with small amounts of salt. Freshwater fish produce a lot of urine.
Saltwater fish produce far less, and what they do excrete is quite concentrated.