Why does saltwater and freshwater not mix?

Answer 1

Osmoregulation is how Saltwater Fish breathe in salt water. They can actually live in fresh water, but would be incredibly stressed. The better question would be why cant fresh water fish live in salt. Marine fish evolved to retain and release salts in their body to keep their body salinity level perfect no matter what the water conditions are. Whereas a Freshwater Fish, can not adapt to salt because it takes on the salt threw the gills and can not release its own salinity level to balance. So their bodies literally hemorrhage from the high salt level

Answer 2

Because ocean is salted water so, the freshwater fish can't live to it because their gills can't breath on ocean.

A little more...Osmotic pressure is the reason. If there's more salt (for instance) on one side of a semipermeable membrane than on the other, the natural tendency is to equalize across the membrane, so the same concentration of salt appears on both sides.

In the case of fresh water fish, their blood is saltier than the surrounding water, so the fish's digestive system spends a lot of time eliminating waste products in the fish's body that are saltier than the surrounding water, in order to reduce excessive osmotic pressure at the gills (a typical semipermeable membrane).

Freshwater fish urinate, for instance, nearly constantly.

Salt water fish have the opposite problem. The surrounding medium is a lot saltier than they are, so the tendency is to equalize the other direction.