Fish use their gills to breathe by extracting oxygen from the surrounding water. The oxygen that fish and other animals with gills use is bonded to hydrogen molecules and is dissolved in the water. Water molecules are composed of two parts hydrogen and one part oxygen.
Fish are able to survive on less oxygen than humans because they are cold-blooded animals, even though water is much denser and subsequently more difficult to pump than air. Water contains significantly less oxygen when compared to an equivalent volume of air. Animals that breathe air are incapable of extracting oxygen from water due to both a lack of surface area and an inadequate lining of the lungs.