Here are some steps for preventing an outdoor water tank from freezing:
Step One - Insulating to Preserve Heat
Make sure your tank is well-insulated. Using insulation with a high “r” value will create the best insulation possible. You may find insulation blankets that are specially made to fit water tanks, or you may have to wrap strips of insulation yourself. Use the appropriate tape to secure the insulation.
Any sides near exterior walls should have extra insulation.
Step Two - Adding Heat with a Heating Device
Timed heaters and lamps are available for tanks in enclosed areas. If you wish to use a timed heater or lamp, be sure that there are no flammable materials nearby and check the heater often.
Another solution is heat tape, which can be wrapped around a tank. Never install heat tape over itself, or put it under insulation. Only buy tapes with 3-prong grounded plugs, and change them frequently – they can be a fire hazard.
Step Three - Heat the Water
If your tank is near a source of electricity, you can also buy a submersible electric heater that can warm the water from within. They cost between $20 and $70.