"Fresh water" and "salt water" are relative terms. All water contains some salt and falls somewhere along a salinity gradient. Every species of fish (and every other aquatic organism, for that matter) has its own, unique range of tolerance for salt (or lack thereof). Osmoregulation (maintenance of appropriate osmotic pressure) is a constant struggle for all of them. "Freshwater" fishes live in an environment that is less salty than they are (hypotonic) and are in danger of losing salt; they compensate by using water freely in their metabolism and excreting highly dilute wastes. Nearly all marine fishes, on the other hand, live in hypertonic environments and are constantly in danger of losing water (hagfishes are the exception; their body fluids have salt concentrations about the same as sea water). Bony fishes compensate by excreting excess salt through their gills. Cartilaginous fishes retain large amounts of organic compounds in their body fluids, enabling them to have blood that's at least as salty as sea water without raising the overall salt concentration of their bodies.
To some extent, fish are able to adjust the amount of salt retained or excreted to adjust to varying levels of ambient salinity. However, they all have their limits. If a fish finds itself in water that's either too salty or too fresh, its first response will be to try to leave. If it's unable to return to a tolerable salinity level, it will die - of dehydration in the case of a freshwater fish in salt water or by overhydration in the opposite case.
Don't forget that some fish travel back and forth between fresh and salt water at different stages of their lives. Anadromous fishes such as salmon hatch in fresh water, mature at sea, and return to fresh water to spawn. Catadromous fishes such as many eels do the opposite. These fishes have to undergo physiological changes, probably regulated by hormones produced by the thyroid and adrenal glands, to adjust their osmoregulatory functions appropriately. These changes are gradual as the fish travel along the salinity gradient; they would die if transported directly from one extreme to the other.
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