Guys will tell you out here that iced over ponds are not the real problem it's snow covered ponds that are! As long as the sun can hit your pond it will produce some Oxygen in it. That's when this becomes a real problem. Lots a guys I know in the north make sure they keep part of their pond cleaned off the best they can from snow for this reason.
RC
P.S. My Grand Father had a pond in Rudolf Wis. As a kid I always remember him running a surface air station in winter. Not sure but maybe a surface air setup would not effect the water as much as far as super cooling it? Plus it kept a 15 to 20 foot surface area cleaned off pretty well.
