new deep sandbed

[gotreef]

Gonna try a deep sandbed and want to know if I can add a 1/2" a week to the existing sandbed to get to 4" or do you think I should remove old substrate. Old stuff is coarse, maybe 2-3mm, and 1"-1 1/2" deep. New stuff will be oolite and like I said going for 4" plus. Now I've read and heard alot of the negative stuff on DSB's but I want to try and if I fail well so be it but I figure at least I tried. So what are the thoughts on the question I have. Thanks for any response.