I winter my pond and have for over 20 years.. in Medicine Hat Alberta.
During the coldest winter the first two feet always freeze, so you must heat your pond with a heater.
I use a 1500 watt cattle heater from Peavy Mart. These heaters are cheap to buy, but costly to run.
My ponds are large (over 3500 gallons) and all flow together thru small waterways. The ponds are over 3 feet deep in some areas.
Aways keep your pond flowing, shut down spitters and waterfalls.
Redirect the water flow to the heater (this is the most important part). This will ensure that the pond is evenly heated and then the fish won't hang out at the heater.
I only heat my pond when it is minus 15 c or colder. larger ponds like mine hold there tempature better. I always aim for 3 inches of ice on my pond. When the water is flowing their is a small open hole here and there.
If your pond is smaller you might find a smaller watt heater somewhere.
I have never lost a fish Thur the winter using this method.
Happy in Canada