Carbon dioxide leaves the tank at the surface and oxygen enters the tank at the surface. This makes water circulation an important function of air pumps.
Water should be really agitated in your tanks. It should “boil.”
Air pumps drive air into the tank. The air either moves water through water filters or it is dispersed by air stones. Some filters do both. It is much better to drive a filter with an air pump than simply an air stone, for reasons we provide in the filter section that follows.
While the air bubbles rising from an air stone can add oxygen to a tank, very little is added this way. The air bubbles actually have two more important functions. Air bubbles travel to the surface of the aquarium and burst, causing small waves to ripple the surface. It is this increase in the surface area and agitation of the water that accelerates the exchange of gases, including oxygen, dioxide and other toxic gases from dead or decaying matter. The second function performed by air bubbles is to create a vacuum as the bubbles travel to the surface. The vacuum causes water to move from the bottom of the aquarium to the top, creating water circulation. Water circulation is key. Without it, the water in the tank soon stratifies into layers and becomes stagnant.