I use sand. It's safer than gravel and easier to clean than big rocks where food can get trapped. You can get pool filter sand (don't ask me where, I asked at Lowes and they looked at me like I had three heads) and that's white so looks nice, and it's cheaper than getting pet store sand. Or you can just get playsand from home depot, doesn't look as nice as the pool filter sand IMO but works just fine and it's cheap.