Not disagreeing about water temps for YOUR particular pond, but anybody else reading the post might have problems in the winter if they have any other fish in their pond besides cold water fish (trout) and fatheads and they placed the diffusers in the deepest part of the pond for the winter. Just remember surviving and thriving are 2 different things, I'd love to see what would happen to your hole size if 2 diffusers were placed 10' apart at 4' below the water surface. Also remember your doming problem? If the lower water was warmer than 33°F and the pond froze over, would bringing up say 35°F water melt the ice any faster than 33°F water?
Moving the diffuser heads into shallower water might help you by creating more wave action - you'd get more CFM from the compressor - what is the pressure/cfm curve for your compressor where they are placed now vs. 1/4 to total pond depth?
There is no thermocline in winter.
In the pond where we have Trout surviving all winter and summer, in winter the diffusers are running at 5' and 8' water depth.
