Ram,

I don't really see the benefit of the "upside down" sand bed. The purpose of a DSB id to create an area of low oxygen for bacteria that complete the nitrogen cycle. That sounds like an oxygenated plenum, or something like a fluidized bed.

If you want to create more oxygenated area, just add a wet/dry. A wet/dry would be a lot less complicated than what you are describing.

Gary