Thanks Heather!
I was told by the lady at the pet store that I needed an air pump, so have been using one all along.
Is it safe to assume that the water being returned from the filter is sufficient then? From everything I have read so far, if fish are not getting enought oxygen they will "gasp" at the surface, right?
Where I have always had the air pump, would it stress the fish if I were to turn it off now?