It can break the glass and heat the viv up, making the temp too hot for the frog. You should use a surface heater, if your room isnt at a warm temp. I just brought a room thermometer for the area my frogs reside in. It was 10 bucks at home depot.
I've had success with the exoterra rainforest heat pads...they are low wattage and shouldn't get too hot. I would be careful spraying the glass where the pad is with cold water though.
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