Yeah, I've gotten mixed reviews on DSB's. They're good for the tank no doubt, but I've heard nasty stuff about problems occurring when its stirred up to much, something about anaerobic stuff in the bottom of it. I don't remember the details. Although this is just hearsay so if someone chimes in with experience with a DSB listen to them because they actually have one.
Although if its a biocube do you really want to take up a lot of room with 4+ inches of sand? You don't have a lot of height to deal with anyways do you?  DSB are probably better suited to non-nano tanks so once again just my .02 leave it as is. 
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55gal,216 watt Tek T5, 45#LR, 45#LS, 3 maxi-jet 900's on a natural wavemaker timer,phosban reactor, 5 gal fuge (built out of standard 10 gallon tank) with chaeto and feather caulerpa, sealife systems protein skimmer, wet/dry, 3 blue/green chromi, 1 skunk cleaner shrimp, 1 coral beauty,1 scopas tang, 2 false percs, 5 mexican turbo snails, assorted dwarf blue leg hermits, astrea snails, and nerite snails |