I've got a 20L that has two 10gal HOB filters on each corner. As long as the water is over the lip, there isn't a disturbance to outgas CO2. I used to run the airline into the filters until I got tired of the annoying sound of the bubbles getting chopped up.
Anyways, I use a wooden airstone ( limewood ) and then I put a powerhead ontop of it, so it sucks up the microbubbles and blows them across the tank. Most powerheads come in smaller sizes, I've got a 660 Pengiun Powerhead which isn't very powerful but gets the job done. I like to have some circulation, keeps dead spots from accumulating mulm and crap.