The freeze is not a good thing when trying to develop a "live soil". I would pile native soil (hay, straw, old insulation, something) around the outside of the pool to act as insulation, and then some kind of insulation on top, but below the tarp. Maybe an old electric heating blanket between tarps?
Does your pools have drain holes? Careful on water collecting on the bottom and creating a "septic situation", or even freezing. My soil is watered every day by timer for 10 minutes by sprinkler, but my soil is on the ground, not in pools.
Good luck and keep us posted.
TMB-