I am using the cheap aquarium air pump, I am in zone 5, temps have between 2
and 20 degrees f. I keep the airtube about 4 to 6 inches below surface and
my water temp stays right at 31 or 32, just don't go to deep or you can
super chill the water. I have hole about 15 inches across all winter, so
far 2 winters now with it in a coffee can turned upside down, works
perfectly, have a heater just incase but haven't had to use it yet....Claude