How Did Salt Water and Fresh Water Fish Survive?

There are many aquatic organisms and marine organisms that have very sensitive a tolerance to to theire, in particular, preferred solinity. So how did salt water and fresh water fish survive in this kind ofye a mixture of waters of different solidities? How did they survive? And it's that just the fish is its own unique set of adaptations. Maybe those critters we see day that have that narrow tolerance are simply inheriting thousands of years of mutations or whatever which rendered them narrowly restricted to whatever environment they find themselves in.