Think Trout, I'd put the diffuser at the deepest point and run from about 1am till 5am to keep the bottom oxygenated, and warm the bottom as little as possible. Much will depend on how many times a day your aeration can turn over the pond.

Keeping the diffuser at 6' is okay for keeping water open in winter, but lets the deeper, cooler water below the diffuser go anoxic during warm months. If there is a thermocline, the trout won't be able to survive in the deep water in summer, no matter the temp.