Bacteria that convert nitrates to nitrogen gas live in an anoxic environment, since they are anaerobic. The DSB needs to be deep enough to create this anaerobic environment -- so at least 6 inches deep, preferably 9 or more. Using a DSB to reduce nitrates is sometimes referred to as NNR (natural ntirate reduction). Otherwise, the easiest ways to remove nitrates are to use macroalgae and water changes.
DSBs used to be more popular than they are anymore. It seems there was a tendency for them to crash after a while, and they caused some tanks to crash. No one is really sure why.
If you are going to use one, it's best to have it separate from your main tank, that way it's easier to maintain. In a fuge is fine. You should still have some nassarius snails to keep the top layer stirred up, but they are not going to get deep down in the sand.