You'll be able to get more help if you let us know the size, shape, and depth of your pond. Also do you have power near by or will you need to go with wind or solar powered? My pond is about 1/3 acre and about 10' deep. I use a 1/3 hp Gast rotary vane with four nine inch disc diffusers in one location. And in the winter I switch to a linear compressor with a stone diffuser in shallower water.
