Fish that can live in both salt and fresh water are rare-can they adapt gradually to a change of water? Or will saltwater fish always die in freshwater (and vice versa)?
You can acclimate mollies to varying levels of salinity, even to a full saltwater tank.
Hadda go look it up.
And Atlantic salmon do not automatically die after breeding; they go back to the ocean. So they go back and forth between fresh and salt water.