Why fish can't breathe air from oxygen?

A fish’s respiratory system is determined primarily by the fact that it spends its entire life in water.Fish have developed gills, on which they rely for the oxygen necessary for a fish’s limited metabolism. The water around the fish produces a tiny amount of oxygen. In order for the fish to be able to stay under the water they need to be able to take in oxygen. The fish's gills do just this. They filter the oxygen in the water and this enables the fish to be able to breathe in water.

If you take a fish out of the water, it will die of suffocation. This is because the oxygen in the air has not been filtered through its gills and the fish can't breathe it in properly.

When you find your goldfish lying dead on the top of its bowl, it has probably suffocated from a lack of oxygen because they oxygen has been used up by some other means such as bacteria. So always make sure you clean Jaws' bowl out regularly!