system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does saltwater and freshwater fish body system work?
gpt
Living organisms need to have some amount of salt in them, but not too much. Now water will naturally want to flow from less-salty areas into more-salty areas to maintain its own balance. The body of a fish will not have the same saltiness as the water it lives in so fish have to do some special things to maintain the right amount of saltiness within their bodies.

Freshwater fish have more salt in their bodies than there is salt in the water. As a result, water tends to flow into their bodies. So they don't drink very much and they pee a lot.

Saltwater fish have less salt in their bodies than there is salt in the water. As a result, water tends to flow out of their bodies. So they drink a lot and don't pee very much.

There are various glands and organs and things that help the fish do what they need to, but that's the basics of it.