General speaking - Don't even try it.
Stick with a Saltwater, or a freshhwater. If you want something in between good Brackish.
Both saltwater & freshwater have some very cool types of fishes.
But don't try to take a saltwater fish an put into freshwater, that's just a bad idea.
Remember it hundreds of thousands(maybe even millions) of years for freshwater rays & eels to convert from a saltwater to fresh, on a long term basis.
Yes, there are certain saltwater fish that are able to survive in freshwater for up to a couple of weeks at a time. And certain freshwater fish that can survive in saltwater for up to a couple of weeks. But none that can go from saltwater to freshwater on a long-term basis.