Sorry for double post but I forgot to say something. I personally like to put 2-3 inches in because my snake likes to burrow quite a bit. Since you use aspen like I do then it'd be good to have 2-3 inches because aspen is great for their tunnels. :-offtopic My friend used to have his snake's viv on top of a metal bar shelf thingy so he could see all the tunnels under the aspen. It looked like an ant farm!