Airpumps to oxygenate the water can't be underwater... where would the air come from? It sounds like a filter to me too. The bubbles go up through a tube, sucking up the water to a small reservoir where it is returned to the tank through a filtering media (sponge, ceramics etc.).
A new tank really should be properly 'cycled' where over a period of a month or so where a good deal of beneficial bacteria is build up to break down the fish's waste. This process can be aided by introducing gravel/sand from an older aquarium to introduce the right micro-organisms.