Rainman is right about the diffuser. For the first year I used the stone that came with the windmill. After reading numerous testimonials, we bought the membrane diffuser. The air stone made nice bubbles but the membrane diffuser made a WORLD of a difference. We now have hardly any bubbles appearing at the surface but the column of water rising up creates the illusion of a deep spring coming to the surface (the color of the water is darker). After seeing the difference, I have to say that the stone is just a bubbler.