text: why is salt water important for fish?   a fish adapted to salt water must have a certain concentration of salt in the water because of osmotic pressure.   the cells of a fish don't have cell walls, just like all animal cells.   if a salt water fish is put into fresh water, too much water will rush into its cells.  as you can imagine, this is not too good for the cell.   some fish are adapted to live in fresh water part of their lives and salt water part of their lives. i really don't know how they do that, but then again, i'm not an ichthyologist.