What temperature are you going to try to maintain. The soil, at some distance below the frost line is a fairly constant 50 to 55 degrees. Immediately under the tank, the soil will try to go to the tank temperature, so if the pond is 70, it will be heating the soil all winter, if the pond is near freezing, then the soil will be heating the pond. The idea of having insulation is to keep nice warm pond from heating all of earth.
Hope this helps you decide on what to do.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard