You put the thermostat probe inbetween the glass and the uth because you don't want the snake moving the probe and making the heat kick on. Put the thermometer probe on the glass under the substrate and set the temp on the thermostat to 90-94 and the cool side should be 80-84 with a hide and the water bowl in the middle. You should not need the lamp unless the ambient temp in the room is below 75! A 20 gallon is way to big so clutter it up with lots or plants and stuff. Also cover the sides and top with black paper, insulation foam(keeps heat in) or newspaper to add security for the snake. I recommend the foam board to help the temps! Here is a great caresheet to help you out: http://ball-pythons.net/forums/showt...ius)-Caresheet