Pick something with a little bit of texture to it to provide traction. Doesn't need to be a lot just don't go with a shiny tile. I have ceramic tile that works pretty well. Others use slate with success. I bought my tiles as 2 - 18''x18'' blocks to fit the bottom of my 18''x36'' tank. I bought three sets so I can pull a dirty set up, clean the bottom, and put a new set in without having to clean & bake the tile right away. I got my tile at Lowe's and they cut them to size for me on site for free. I'm not sure if this is the case at Home Depot though.
