Here we go again!! The use of plenums has been argued thoroughly on this board and if you do a search on "plenum" or "DSB" you'll find a ton of info. I think more people use DSBs because they're so much easier, but many people use plenums with great success also. Check out this link by Ron Shimek which heavily supports the use of DSBs: http://rshimek.com/reef/sediment.htm