I'm trying to get my green tree frog tank set up properly. There is a layer of pea gravel topped with several inches of coconut husk, and live plants. I tried a few lighting/heating arrangements and nothing seems to really get the heat gradient right. I was considering an undertank heater but I don't know if it would penetrate that much substrate or if it would be safe in such a humid environment. For instance, if any cold water should permeate down through the gravel would the glass be hot enough to crack?