Now hang on there! When you put some salt in your fresh tank, you aren't putting in the same amount that you put in a marine tank. And usually you aren't putting in marine salt either. The kind used in freshwater as a theraputic bath is just plain salt, no buffers etc....and there are plenty of freshwater fish that cannot stand even a small amount of salt in their water like discus and tetras. Most tetras will stand it for awhile but will "mysteriously" die at an early age. To be honest, I would not dose a fresh tank with salt on a regular basis. It simply isn't needed if your water quality is up to par. Just wanted to clear that up so that someone doesn't come along and start putting oscars in a saltwater tank!
But, there are other fish that can go from fresh to salt and anywhere in between......monos, scats, glassfish, SOME of the puffers commonly sold as freshwater. Most of these fish will do better in brackish than in freshwater (glassfish), and some will do better in full saltwater rather than brackish (monos).
HTH