I have about an 1" to and 1.5" thickness in my tank............some people say "NO MORE THAN AN 1 INCHOR NO LESS THAN 6 INCES,"their reasoning being "either go thin and have an entirely aerobic bed" or "go thick and have an entirely anaerobic bed"........where others seems to go 2" or 3".........now the way I see it, the top section will always be anaerobic (say 1/2" or so) and anything under than will be anaerobic (all provided you don't have an under grave filter)..........but why do people care if it is aerobic,anaerobic, or both........I would THINK, as long as the grave stays like it is (you don't turn it upside down), it wouldn't matter?