Fresh water does not have enough salt concentration to satisfy the survival of saltwater fish.Therefore the fish's hypertonic cells willbecome hypotonic which will not mix well with the body structure of salt water fish, and they will die. Osmosis helps even out the concentration inside and outside the fish's body. read more
If you put a freshwater fish into saltwater, most fish would lose weight (from losing water from its body) and eventually die. Approximately 2% of all 21000 species of fish actually move from freshwater to saltwater or from salt to fresh at some point in their lives, the move would kill any other fish. read more