Greenhouse Water Tanks -- Deicing?
Another option; use low wattage aquarium thermostat/heater like 25 watt ( maybe more for your location , my coldest weather is only about +20 F). Only turn on power when a few days of freezing weather is expected. If the power is left on a few days extra, the low power usage will not be high. The hoses need to be insulated as they will be the first things to freeze. The main concern is the surface of the water will freeze and build up pressure; For pools an inflatable bladder ( such as a tire inner-tube ) is anchored at the surface. Then the surface cannot make a pressure tight barrier permitting a pressure build up in the water.