system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why can't fish that live in salt water survive in freshwater?
gpt
Osmosis is a process in cell membranes under which water goes from areas of low solute concentration to high solute concentration.

Organisms generally can't live with lots of salt inside them, so saltwater fish have low levels of salt inside themselves, but are surrounded by water that has high concentrations of salt. So they are constantly losing water through osmosis. As a result, they are built to constantly "drink in" more water (and have their kidneys filter out the salt).

Freshwater fish generally have a higher level of salt inside their bodies than in the surrounding water. They are constantly taking in water through osmosis. They are therefore built to constantly "pee" out excess water.

Since most saltwater fish are built to deal with osmosis in a way that is *opposite* from the adaptation they need, they cannot survive in fresh water. They do not have the same adaptions to rapidly get rid of water from their bodies, since they are used to water naturally leaving their bodies through osmosis.