I use paper towels for my leo now, but I've used slate tiles in the past. I didn't seal them in, would just wipe them down and periodically (usually on a monthly basis) pull them out and clean both sides completely as well as the bottom of the tank. The slate looked nicer than the paper towel, but I think I prefer paper towel. Clean up is way easier, and as long as you've got good heating elements and thermostats, you'll be fine with heat retention.