We've got a fairly aggressive tank and I know what you mean, those guys can EAT! Some of our livestock is similar to yours (we have a porcupine puffer too and a couple of voracious triggers). The crabs will be a snack sooner or later. We found that having a kole tang and a couple of angels in there keep it pretty reasonably clean. The kole tang is in the family of bristle tooth tangs, so they really graze on the rocks and so does the angel.

I do have to wonder if you are feeding too much though if they just let it fall to the sand and none of the others are gobbling it up? Or maybe it's a matter of not feeding the right foods or the wrong particle size?