Richard F. answers this one for you.
All animals, including fish, have to breath in oxygen and breathe out carbon dioxide. Many animals, including all mammals, use lungs to breathe. Lungs only work in air. If they fill with water, the animal drowns. There is oxygen dissolved in the water, but the lungs are not able to get it out.
Fish have a different way to breathe. They use gills, which are in slits on the side of the fish's head. The gills get oxygen out of the water that flows past them as the fish swims. If you take a fish out of the water, it suffocates, because its gills are unable to get oxygen from the air, just as our lungs are unable to get oxygen from water.
The Compton's Encyclopedia has this picture
A heart pumps the red blood through the body of a fish, just as it does in higher animals. Most fish breathe by means of gills. They consist of many tiny filaments suppliedwith blood vessels.Water enters the open mouth. Then the fish closes its mouth and the water is forced over the filaments and out through the gill covers. Oxygen dissolved in the water is absorbedinto the blood stream through the delicate membraneof the filaments. Inside the mouth are straining devicescalled gill rakers. They prevent food from passing over and injuring the gills.% % filename IO.DOT
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