Mssr. Billp..If you are in search of a long term solution to this problem, perhaps the following suggestion will be of some value.
Construct a thermally engineered structure over or enclosing this tank with as high an R rating as practicable.
If in your latitude and climate
passive solar heating has merit, than it too might be considered in the overall plan.
With a little imagination the outer configuration could be tweeked to reflect any aesthetic considerations for that locale.
The co-generation of heat and storable electrical energy could be accomplished with a tiny little diesel generator in the event of prolonged cold weather.
Many blessings on your family and may your oasis never freeze.
Akmed..of de Nile