Crushed coral will eventually work its way to the top. One of the big cons of crushed coral is that it traps a lot of detritus and must be 'vacuumed' regularly. The sand would be on top for a while, but eventually you'd end up how you started, but with an elevated level of cc. I started w/ cc in my tank and now I wish I had a dsb, but I didn't know about those when I started. Now I just can't bring myself to tear my tank apart to put in the dsb. I move every 3-4 years anyway, so I figure the next time I move I'll set up the dsb.