Because the concentration of salt would “dry” their cells out. That is freshwater fish in salt water.
On the other hand, cells in saltwater fish have mechanisms for which this process does not affect them. However, if they go into freshwater, the same mechanism would bloat their cells and kill them.
There is also a problem with ammonia disposal, or fish pee if you will. Due to osmosis, freshwater fish would have a problem releasing this waste and would eventually die from ammonia toxicity.
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