Sounds like she's able to bury herself and get her heat as she needs it. To help up the overall temp in the tank try wrapping it in insulation to help hold in any heat as it builds up. Glass is an excellent heat sink and will dissipate the heat really quickly. Having the top not covered at all is also a loss of heat, I would try putting a piece of plastic sheet over at least the portion of the tank the heat is on, leaving some of the screen open to allow the humidity to escape. ----- Reptiles Make The World Go Round.
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