Hi,
I've always avoided using incandescant light bulbs to heat dart frog terrariums because they tend to dry things out. If you do use one I would go for an infra-red one that can be left on all of the time. I think 40 watts would be too high of a wattage and a 15 or 25 watt bulb would be better, just make sure to do temperature checks.
Often times the heat produced by the flourescant lighting is enough to heat the terrarium. On the few cages of mine that do require extra heating I use heat pads but stick them on the side or back of the terrarium behind the background rather than under the tank. This works pretty well and usually heats and area of the tank up by 5 or 10 degrees. Good luck,
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Devin
devin@amphibiancare.com
www.amphibiancare.com