7 ft. is only 3 psi; most any small air pump will deliver at least 1 cfm at that depth. I have a gast I got at a surplus store for $55 bucks. It has no problem supplying my homemade co-active diffusers(sorry, Vertex) to 12 ft. It is 55 watts; much more efficient than moving water.
If I didnt have it, I would look into an air brush compressor, which is oilless, rated for continuous duty, and not expensive. This is if you have no power at the pond.
I also have a piston gast, that kicks butt, but I have it off duty to overhaul it.
