think about the aquatic environments as well as the terrestrial ones. An adaptation to salinity explains why saltwater fish cannot survive in fresh water and vice versa. How do fish that can survive both environments adapt?
All living organisms have to maintain a certain level of a substance in order to survive. This is called homeostasis. Salt water fish need salt water to live because their cells have adapted to high concentration of salts being around them. Becasue they are in a high conc of salt their cells inside do not need as much water. The opposite is true for fresh water fish. If you put a salt water fish into fresh water the fishs cells would take in too much water and eventually burst. If you put a fresh water fish in salt water it would become dehydrated.