That's an important question! About 21% of the air we breathe is composed of oxygen. Even though fish live in water, they need oxygen too. Unlike us, fish do not have lungs, but instead uptake dissolved oxygen from the water using their gills. The concentration of dissolved oxygen in water is much lower than the amount of oxygen in the air.

Fish do best in water that has dissolved oxygen which is at least five parts per million (=0.005 grams of oxygen in 1 liter of water). If the dissolved oxygen drops below about 2 parts per million, most fish will suffocate and die. Therefore, it is important that lakes, rivers, and streams remain relatively oxygen-rich. Things that can affect dissolved oxygen concentration include: temperature (colder water will hold greater oxygen concentrations) and pollution (things like sewage in water can cause blooms of bacteria that use up the dissolved oxygen, hurting fish).