I started with about a 2 1/2" deep bed, and later got a diamond goby to help keep it clean (and a wonderful job he does I might add). But the goby moves the sand around to his liking, so now there are a couple of corners that are almost bare bottom, and some areas in the middle that must be 4 1/2" deep. But he keeps all areas sifted fairly well, so there is not much buildup of bad stuff anywhere.