I have two tuxedo urchins in my 72 gallon bowfront. As mentioned earlier, they tote stuff around and do some minor re-arranging in the tank. Mine have cleaned off most all of the coralline on my lr
. They keep whatever algae I might have on my glass (which isn't much) cleaned off. I put out a couple of sheets of algae for the tangs and another for the urchins.
As difficult as it might be, perhaps you can review your feeding schedule and cut back some. I had this problem early on, until Roka mentioned "you don't have to feed your fish everytime you walk by the tank, and they come up to beg for food."
I'm para-phrasing here but that's the jest of his comment.