Hondo,
Trust me the small air bubbles will work just fine. Of course the deeper the diffuser the better it will work. However do not put my diffuser(s) in the deepest water in the winter. I set them off to the side as I do not want too vigorous of mixing of the water column. I have found that can stress warmwater fish which seek the warmer water in the deepest part of the pond (relatively speaking).
