How Fish Drink Water – Proper Aquarium Osmotic Function
By Carl Strohmeyer
ANSWER: Freshwater fish absorb most of the water they need through their skin via osmosis (is the net movement of water through a selective permeable membrane from a region of low solute potential to a region of high solute potential), not through their gills. The gills are for respiration.
Saltwater fish actually drink the water the live in, as the salt in the water is constantly pulling H2O from their bodies in a reverse respiration, this is why some fish such as catfish are sensitive to salt in the water, but this is also why some fresh water fish are helped by salt to generate a mucous slime coat on their skin which is necessary for disease prevention. Proper electrolytes, magnesium and other elements are important for slime coat generation.