In the back of my notebook I have a partial design for a passive compost tea pump. The gist of the idea is a large steel frame with two sealed 55 gallon drums mounted on a lever that could be locked. The air pumping action would come from water in the upper drum leaking down into the lower drum and displacing the air inside, once it drained fully you would jack the full one up and the now empty one is on the bottom having the air displaced. The operator would switch the position of the drums to keep the pump constantly running. The details of the actual hardware and mechanics are rough at this point, but I imagine a jack/lift or lever mechanism to make lifting the drums back and forth easy enough.
The general concept of the air pump is demonstrated decently on this youtube vid
More in the spirit of this thread I commented 2 years ago that the ecoplus 1 was a good pump, I still think that, although it is loud. I have since been using an Ecoplus 3 and its been good, but loud as well. It is suitable for 50-100gs.