From what limited knowledge i know about BB tanks, they employ high flow and water turnover rate to lift whatever wastes in the tank into the overflow. That prevents the wastes from breaking down, thereby controlling nitrates. Theres some pros and cons in employing a BB tank though, do a serch on the forum, theres lots of thread rdg this topic. For me, i still prefer a layer of sand, more natural and if you don't have sand, you won't be able to keep certain fishes.