Bill You are correct on the 4- sided airstones. The Great Lakes ALA4GL (picture in Jan/Feb PondBoss pg 35)has a approx 18 X 18 inch underlay for the stones and I have not wittnesed any additional turbitity on these.I would still like to see them 4-6 inches off the bottom. The round Koender type which has a solid bottom and is a different story and is best suited in a 5 gallon bucket to create an updraft. Our Vertex Airstation (pic in action on installation page)is mounted 7 inches above the top of the weighted base and 11 inches from the bottom of base and is designed to sink into muck (organic material) only 4 inches to the lip at the top of the weight box. Something to consider when installing a diffuser into a mature pond bottom.I have replaced many airstones in buckets that fell over at some point even with pea gravel in the bottom of the pail.If one is going to use the airstone bucket method consider drilling 4 holes in the bottom of the pail and inserting 2 pcs of 3/4 to 1 inch pvc to create an X 18-24 inch works.Add 2-3 inches of pea gravel and I would add a float just under the surface so you can find it when it needs to be removed for cleaning. The diffuser must continue to float down as the organic material decomposes. (sorry about getting off the original post)