Ignore my ignorance here please

[mark]

start here http://en.wikipedia.org/wiki/Osmoregulation

[SeaShell]

OMG! That's so interesting!

[Matt]

And, some fish have the ability to move between marine and freshwater environments. One very common example is Salmon. Another is the common Molly, which can live quite comfortably in either, and switch back and forth.

[justinl]

Although those that switch from fresh to salt or vice versa have to undergo some pretty intense internal physiological changes and must be acclimated. You cant just dump a molly in saltwater and expect it to live long.