I was talking to a friend yesterday and I used the phrase "You drink like a fish" and I guess a lot of us know the meaning which implies a person takes a lot of alcohol till they get drunk or sober. So if we drink alcohol and get drunk only to pass it out through our urine, what about fishes.
For the sake of clarification, not all fishes drink water so we could say the phrase is looking at some particular type of fishes because fishes in fresh water do not drink water while those in salt water drink a lot of water. As living things we cannot do without water, we just need the right amount of water and although the amount might differ from organisms to organisms, we need it for survival. Now that we know, let's focus on salt water fishes.
The gills of fish is responsible for allowing oxygen into the bodies of fishes as well as allowing them out of the bodies of fishes. Because the fish is in a salty water doesn't mean it will have equal concentration of salt as the water. The inside of the fish is not as salty as the ocean water so water moves out of the fish to the ocean just to balance the concentration in the process known as Osmosis and this can be dangerous for the fishes themselves.
In other to balance things through homeostasis, the fish needs to do something. Just like we pass out water through our kidneys when we drink them in excess and drink water when we are thirsty, so does fishes find a way to solve their problems and the fishes drink the water in the ocean so as the get the oxygen they need and not lose the entire water in their body.
If they continue to drink this water, then their salt level should increase but this doesn't happen because because they use chloride cells in the gills to pump out the salt from their body. For ocean water fishes, their urine is salty and quite little. For fresh water this is completely different since their body is salter than the water they live in, so water is continually going into the body through a process of osmosis. So unlike salt water fishes, fresh water fishes need to urinate a lot so as to release water else their cells burst.
The urine of fishes is made up of phosphorus and ammonia which is very useful for algae and coral. Some fishes like Sharks have Urea packed in their tissues so as to prevent water from getting out of their body in the salt water so they do not have to continually drink the salt water. But there are some fishes like Salmon that can live both i fresh and salt water, as they are able to twerk their body to stay in both. Just like fishes, we are also able to regulate our salt water balance in our body and researchers have agreed that we got those abilities from our ocean ancestors.
https://cafishvet.com/fish-health-disease/do-fish-pee/
https://www.smorescience.com/do-fish-pee/
https://med.libretexts.org/Bookshelves
https://www.vaia.com/en-us/textbooks
