i dont know what size tank you have, 75? but i have the same one. friends told me that rocks wont crack the bottom but just be careful about it. one thing you can do is go to lowe's and buy fiberglass rods and cut them, drill holes in them and stack them like that
youll like it a lot better without sand, since you wont have to worry about the flow. youll probably even want to add more of it to keep detritus suspended so it can be caught in a filter sock, or at least push it so itll collect in one corner and you can siphon it out