Whoa, have you any fresh water experience? I say this because your question is the very basic of understanding water and the ability for the fish to breath oxygen. You see the transfer of oxygen to water happens at the surface not through bubbles in the water. The more distrubance at the surface of a body of water creates more oxygen to that body of water.
So no your question isnt stupid. As well, a air pump is never necessary, although if it is your only form of surface agitation then yes it is necessary to create oxygen in the volume of water you have I.E your tank.
Scuba steve made a great point a battery operated one is good to have during a power outage, but then again you could just stir the top of the water to get an oxygen exchange every 5 mins.