Why do fish swim drink and pee in their own pee?

Well, they can't just change to a new bowl each time they have to urine or drink. All they eat is fish food. Fish food has nothing bad inside of it. With that being said, the fish can drink it's pee because all that is in its pee is water and unneeded parts of food that were in the fish food. That's your answer. Fishes aren't stupid and either way if you take the fish away from any food cycle such as:

Vegetation

Small Fish

Shark

What dies? The shark. I know that wasn't the best food cycle ever but I just wanted to state my knowledge on fish. Haha.