Gast Rotary Vane Pumps will not lose very much volume, if any even at their max depth of 18 feet.(Actually it is more like 20', but we tell our customers 18' just to make sure they do not place the diffusers too deep) Here is a resource for anyone looking for brand new Gast Rotary Vane Compressors:

http://www.livingwateraeration.com/aeration-compressors-gast-rotary-vane-compressors.html

Also Rocking Piston compressors are an excellent alternative. About a year and a half ago we began to carry Stratus Rocking Piston Compressors. These compressors have several advantages of their Gast counterparts. They produce more volume, they use less electricity, They can operate at greater depths (50' Max) and they have an additional year of warranty. Here is a link to these compressors for your reference:

http://www.livingwateraeration.com/more-items--stratus-rocking-piston-compressors.html

Daniel Crackower
Living Water Aeration