Why can’t saltwater fish live in freshwater, and freshwater fish can’t live in saltwater?
Their bodies are not able to work in the water type they weren’t raised for. Specifically, there’s a process called osmosis that causes water to move across a barrier (like the cell membrane) in the direction of more stuff (salts, proteins, things that want to dissolve in water). So in freshwater fish, their cells have special systems to keep the water from flooding into their cells (without them, the cells would pop like a balloon). Saltwater fish have systems that help them deal with the incredible amount of salt they live in, and the constant pull of that salty environment on the water inside their cells (without them they’d shrivel up like a raisin).
Imagine it like air.. we evolved in Earth’s atmosphere so we breathe oxygen. If we go to a planet with a primarily nitrogen atmosphere, we would not be able to breathe it.
Specifically for freshwater fish, I believe they get dehydrated by seawater (the same way humans do if they drink it).
I don’t know the science behind it but a salt water fish is evolved to live in salt water. Putting it in fresh water is like putting us on the top of Mount Everest. It’s technically air up there but it’s so different from the air we normally breathe we aren’t adapted to use it.
Salt in high amounts is toxic to most life. For life that is adapted to live in salt water a fresh water environment would be toxic to them.
Freshwater fish will get their water sucked out of them into the salty ocean, scaly fish in the ocean have them to protect from their water being sucked out. Other marine animals without scales like octopi or squid fight this by filling their blood with urea, resulting in a isotonic relationship so water doesn’t try to flow from low concentration of solutes to high concentration
If you fill up a bag with salt water, tie it shut and then place it into a bowl of freshwater, the saltwater will leak through the bag into the fresh water. (some kind of weird physics called osmosis, that makes it so the salt wants to spread evenly and makes it so it’s, somehow, able to pass through the bag. Don’t ask me for further details on that.)
From my understanding, this is basically the same thing that’ll happen to salt water fish is placed into fresh water. The salt in their cells will kinda leak out. Since they’re made to have a certein salt content in their bodies, they will die if they don’t have that.