Hermit Crab Question

Hey, I am getting hermit crabs on tuesday ( probably two) and I am setting up the tank tomorrow. I just have one question though. For heating, so far I don't have anything except for the regular light in the hood. I was told by the pet store that this will do just fine for heating the tank...but what about at night? then wouldn't it get cold? They sold under-tank heaters for hermit crab cages and I am wondering if the light will do just fine or if I should get the heater, but if I have the heater then should I use the light? I've heard the light can be drying, but if it's a source of heat...
Can someone help me?
Thanks

Thanks[/QUOTE]Well I have 2 hermies I jave a 10 gallon aquarium and as for heating I use a undertank heater,I didnt want to use the lights beacause I was never sure how many watts.... because some are too strong so I just stick with the UTH :D