Check out the reason for the random facts why freshwater fish cannot survive in saltwater at right here
Basically it’s the same reason why you would die of dehydration if you drank salt water instead of fresh. When you have water with differing levels of salinity and a permeable membrane separating them, then the water will exchange between them to try to reach equilibrium. If a freshwater fish were put in salt water, the extra salt in the water would absorb the water from the fish via its gills until it died. The reverse would happen to a saltwater fish placed in fresh water; it would absorb too much water through its gills and die as a result.
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Some fish can live in both fresh and salt water (salmon, eels, bull sharks for instance) but they require time to adjust between the two. Most fish have not evolved this adaptation because the situation rarely presents itself under the situations where they live.
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