Why is it that salt water animals do not survive in fresh water environment?

The various species of fish found in oceans, lakes, rivers and streams have evolved over millions of years and have adapted to their preferred environments over long periods of time. Fish are categorized according to their salinity tolerance. Fish that can tolerate only very narrow ranges of salinity (such Freshwater Fish as goldfish and such Saltwater Fish as tuna) are known as stenohaline species. These fish die in waters having a salinity that differs from that in their natural environments.

The only fish I know of that lives in both is Pacific salmon. Salmon are born and they reproduce in the safety of freshwater but spend the rest of their lives in the oceans. This type of fish (fresh and salt water) are called anadromous.