Either one. My tanks have carpet (our house carpet, not repticarpet) and non-adhesive shelf liner on top of it. This is to keep the carpet from smelling to bad when they poop on it. This is kind of funny as they don't seem to poop very much. When we first got Zoom Zoom, we bought a package of paper towels from Costco because we heard they go through a lot when they poop on them. Well, Zoom Zoom doesn't poop as much as speedy and she tries to figure out a way to not poop on the floor. We were at my brother's place for thanksgiving and she pooped on the stick she sits on, instead of the floor. Silly girl, that way you tend to sit in the poop. Well, at least she's pooping.