Fish consume the water they inhabit if it agrees with their system. Since the bodies of freshwater fish are much saltier than the water they live in, they don’t drink.
They are in danger of too much water entering their bodies, diluting their body fluids. They urinate often to get rid of any extra water that soaks into their bodies.
Saltwater fish, however, drink large amounts of water, since the salty sea water absorbs moisture from their bodies, causing dehydration. These fish eliminate excess salt through their gills and very concentrated urine.–Dick Rogers