That sounds like it will be a pain to clean (because the combination of gravel and tiles will trap poop, leftover food, and dead plant parts).
Gravel needs to be vacuumed as regularly as water changes--once a week. I would get it out of there because it probably won't be full proof. Axies like to redecorate other things, including big rocks; they could still get under the tiles.
(Message edited by j4782 on January 18, 2006)