Fish need oxygen, but since they do not have lungs they get their oxygen from the water they live in. The oxygen in the water that fishes use is called dissolved oxygen, since the oxygen is dissolved in water.
Fishes use their gills to get oxygen from the water in a way similar to the way humans use their lungs to get oxygen from the air. Air has 21 percent oxygen, while oxygen in water is a rare resource. Fish only need a tiny amount of dissolved oxygen. They are able to live well if the dissolved oxygen is at least five parts per million. If the dissolved oxygen is less than two parts per million, most fish will eventually suffocate and die.