Garters do need some kind of heat - for digestive purposes. I agree with Shannon and have an under tank, very small, heater. Now, granted, my garters don't sit on the rock as they burp their meal... But they do have a thermal gradient that allows *them* to find what they need for digestion.

I have no AC, so in the summer I usually don't use my under-tank heater. Not necessary with temps in the house upwards of 80 degrees.

I also have a white light to give additional heat at one end - the "hot spot" - during colder times. And I have a red light (60 or 40 watt plain red bulb - NOT A SPOT LIGHT!) - for colder nights that get down to 50 or 60 degrees and I don't want to close the windows yet.