Likely its detritus. I don't even have a sandbed, run a 100um filterbag and its particle city in my tank. I also run ozone and it helps break down some of the bits but every once in awhile a fish will stir up some crud thats hanging out in the back forty and its snowglobe time. I also siphon detritus on the bottom weekly. I don't really know what to suggest other than that ozone does help (at least I think it does in my tank).
BTW with all those tangs in the tank they're more than likely creating a lot of, um, detritus (poo) so I would imagine its probably difficult to keep on top of unless you want to feed them less. Perhaps try a filterbag that is a smaller micron size. You will have to swap them out more often but it might help.