Like Larry says you just connect the air pump out-let to the venturi on the powerhead, rather than an air stone. I use a 4" x 4" x 4" centre bored PPI10 sponge (They sell them at Koi carp places, I usually buy a 12" x 4" x 4" and cut it into 3, but you can buy them as 4" cubes etc) and a rigid drilled pipe siliconed to a plain white glazed 4" tile. The advantage of these is you can just tuck them in a corner.
Have a look here for the set up (from the excellent Loaches on-line):
<http://www.loaches.com/articles/a-river-runs-through-it>.
cheers Darrel