I have a 28g Nano which has a sand bed of about 3-6 inches, theres also a 10g Fuge with out any sand in it(This might change). The reason for my DSB in the nano is because it being such a small tank changes can happen very fast so having somthing with a large amount of BB means that I hopfully will never have my tank crash on me again (Nock on wood). Sometimes with larger tanks because of the amount of circulation and live rock a DSB can just cause choas although, you find alot of people now a day just plumping up a 150g rubbermaid container with a 6-8inch sand bed just to stabilize the display tank
.