Z, do you have plant life in the pond? If you keep the ice cleared of snow in strips and have plants, you have far less risk of a winter-kill. If you use dye and the pond doesn't have much plant life at all, I would suggest moving your diffuser to a depth of about 3 feet and leave the windmill running all winter to keep at least a small section of open water.

Also, if your pond is fairly young, under 3 years old, you have a lower risk of a winter kill.