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.
Another note, because most freshwater fish cannot drink their surrounding water (Salmon and others are exceptions), when you place these freshwater fish in saltwater, they actually dehydrate.
Osmosis in fish;
There cells must always be bathed in a solution having the same osmotic strength as their cytoplasm. This is one of the reasons why fish and other animals have kidneys. The exact amount of water and salt removed from their blood by the fish kidneys. The process of regulating the amounts of water and mineral salts in the blood is called osmoregulation. Fish which live in the sea (remember the sea is full of salt and other elements), but fish which live in freshwater have the opposite problem; they must get rid of excess water as fast as it gets into their bodies by osmosis. Osmosis is an important topic in biology because it provides the primary means by which water is transported into and out of cells.
Generally salts, not just sodium chloride can affect osmosis. Magnesium can also play a major role too. Calcium can affect and just as importantly be affected by proper osmotic function. It is important to have a positive Redox which describes the ability for the loss of an electron by a molecule, atom or ion to the gain of an electron by another molecule, atom or ion. Without this positive Redox Potential many minerals cannot be absorbed and properly assimilated. So it is very important to keep a “positively charged” aquarium or a high Redox Potential (300 -400mV) via high dissolved oxygen levels, UV Sterilization, and proper electrolyte levels. For more information about the Redox Potential, please see my article: http://www.americanaquariumproducts.com/Aquarium_Information.html