I am in zone 5 and I used to keep my 100 gallon deck pond going all
winter with a submersible aquarium heater and my pump bubbling water....
not only was it above ground, it was 10 feet above ground. I just added
water frequently due to loss. From Jan - Mar I had about 2 inches of
ice except where the pump was... that really helped stop all the water
loss from steam. I would still do it but they switched to Chloramines
this years and it makes it a bigger pain in the butt.