Fish need oxygen to live, and they get the oxygen from the water. So, in a standing container of water I'll need to get an elaborate system of pumps. Luckily, I'm wrong again.
I can certainly get an air pump to oxygenate the water, but if I get a good filtering system that promotes the movement of the water and agitates the surface, I really don't need the pump.
As my aquarium grows (hopefully!), I may need to readdress oxygen and other requirements necessary for the health of the inhabitants. But for now, I should be OK without an additional air pump.
Now, another complicated piece -- click "I'll Catch Ammonia! Or, hit Let's Get Some Fish! and go back to the beginning.