Anadromus fishes are those who enter freshwater lakes/rivers to spawn…salmon are a good example. Somehow, these fish have learned how to live in fresh and saltwater environmnets. My question: can you accustom any freshwater fish to salt water? Say goldfish, if you bred successive generations of goldfish , all the while gradually raising the slat concentration of their envuironment, would you eventually have a solt-water goldfish? I ask this becuase it seems that many fish are able tolive in bith-what’s the big trick about this?
I’d like to breed saltwater piranha fish!
I would say “yes… if you have the time”.
Way back, oh, I don’t know - but not long after bony fishes started evolving, so around the Devonian - we see fishes in the fossil record from freshwater environments. (I’ve never studied ostracoderms intently so I don’t know if they were ever found in FW).
It is believed that these guys started in marine environments and simply began to take advantage of the less dangerous places upstream. Add many years (I’d say at least more than twenty 
) and they begin to express adaptations to FW (cellular salt concentration, etc) in their populations. Some species were just able to keep these traits.