Can you move the diffuser nearer shore, into much shallower water? That would increase the safety and accomplish the mission of aerating enough of your pond to keep your fish alive. Plus, aerating water that's three feet deep allows nature's forces to keep your bottom water several degrees warmer, since water is its most dense at 39 degrees F. Depending how close you bring the diffuser to the edge, that should determine the size of the hole you have in the ice.

Teach a man to grow fish...
He can teach to catch fish...