Hey Kevin,

I would agree with you as far as natural heating goes, it would be from the top. But the soil would be warmed from the sun. I have had great success with keeping a heating pad on low under the earth end of the tank. The other end is a pool of water. I use fluorescent lights that really do not provide any heat at all. So, I have compromised on delivering heat. They grow and digest.

It works for me.