system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't fresh water fish survive in salt water?

Explain like I'm five, why can fresh water fish survive in salt water and salt water fish survive in fresh water?
gpt
It's the same reason you can't drink salt water: osmosis. Low solute water will flow into high solute water (the solute being salt). Salt water fish have different body chemistry than fresh water fish. So salt water fish lose their body's water to the sea, and will drink sea water to replace it. Fresh water fish, on the other hand, have excess body water and are always excreting (urinating) the excess. 

Basically, salt water fish = water flows out of their body. Fresh water fish = water flows in. If you put a fish in the wrong type of water their body isn't equipped to balance their body's water content.