I find these to be excellent herbivores - much better than snails

They are reputed to eat corralline algae and "One researcher put urchins, herbiverous fishes such as tangs, and algae together in tanks in various combinations. The results were interesting and unambiguous. Sea urchin herbivory promoted the growth of corraline algae in almost all situations, while the presence of herbiverous fishes and oscillating flow regimes promoted the growth of filamentous green algae("hair algae")" (Ronald Shimek Ph.D., Without a Backbone, Aquarium Frontiers, 6.6.99, p5)

For some unknown reason all my urchins died.