What were to happen if large amounts of salinity...

were to suddenly appear in freshwater? what would happen to the fish? and if all the salinity in saltwater were to dissapear and become like freshwater?

I know its a strange question but somtimes weird things just pop into my head like this... (LOGICAL answers please no stupid irrelivant trash or comments like "what an idiot" just to get 2 points please people..)

Thanks!

Logically, those plants and animals that have adapted to fresh water would die if the salinity were to greatly increase, and those plants and animals that have adapted to salt water would die if the salinity of the water were to drop to zero.