I am just getting into the hobby & have larger tanks, but I would like to set up a little 20g in my office (@ home) with the goal of a nano reef down the road. I have seen/heard that a DSB should be appx. 4-6 inches deep. Does that apply to the very small tanks too? Putting a 5" DSB in a 20g tank seems like it would take up a lot of space...but perhaps it requires that depth to accomodate both aerobic & anaerobic bacteria? Any thoughts? Thanks!!