to many in such a small tank not enough food for them all. YOUr tank levels might be out of whack. I would find a new home for a couple of them I have 1 small tuxedo urchin and he does a good job on his own. I also have 1 slider conch which is doing a good job of turning the sand over it will bury it's self in the sand and move along turning it over as it moves then it will come to the top and move along the top layer.

Bill