Why do fish need salt water to live?
Salt water fish need salt water, fresh water fish need fresh water. Humans need air. Its just the environment that it grows in.
Saltwater fish require salt water, and fresh water fish need fresh water, you can't mix them, or the fish will die. You need to look at each individual fishes needs as to what temp they need their water to be. There are some saltwater fish that can survive in fresh water, but it is not healthy for them.