i use newspaper on mine. if you use aspen you'll have to crank up the heat a little higher than with newspaper or something thinner. if you leave the t-stat at 92 on the outside of the tub, itll be 90 where the snake is touching on top of the substrate. if you use aspen, depending on how thick you make it itll be a few degrees cooler so you'll have to crank the temp up a couple more degrees to compensate. these numbers are jsut random BTW and just an example.