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, and vice versa?
gpt
Many fish can, and do.  Salmon, for example, make babies in fresh water.  The babies swim out to sea and grow up and get big and strong, and then they come back to fresh water and make more babies.  They have this ability because over millions of years, they were able to have more babies in freshwater than saltwater.  That is, they evolved to take advantage of reproduction in a freshwater environment.  The fish that can't do this made babies just in the ocean or just in freshwater, and so never had pressure to evolve to be able to tolerate both environments.

A fish's body wants to have an amount of salt inside it somewhere in between the ocean and a river.  The way both kinds of fish's bodies do this is called *osmoregulation*.  So a freshwater fish will get its salt from its food, and it makes its pee super watery, or diluted, so there is more salt inside than in the water.  A saltwater fish gets too much salt just from living in the ocean, so it makes its pee super salty, or concentrated, so its body is less salty than the water.  Most saltwater fish can't live in freshwater because they only knows how to make their  pee concentrated, so they'll lose all its salt.   Most freshwater fish can't live in saltwater because they only know how to make their pee more dilute, so they'll get too much salt.