This is a very good question, and the answer is kind of a trick. Although fish do breathe the same molecule that is found in air (oxygen), they cant actually breathe the air! Fish have gills which effectively filter dissolved oxygen from water. In air the gills just don't work properly. You and I have a different kind of breathing system that takes the oxygen gas from the air and diffuses it through our lungs into our bloodstream, and that's how we get oxygen. But if you put us underwater, our system won't work, even though there may be a lot of oxygen in that water! This is an example of how different organisms can live in different environments depending on how their bodies work.