Tuxedo urchins.
Short spine and rock urchins from the Caribbean, Royal Urchins (Tripneustes gratilla). Do not get Pencil urchins, majority I've had over the years will chew on some species or variety of coral sooner or later. It's hard to believe for an animal with no brain as we typically think of, but I've seen what appears to be individual behavior and prefferences in different specimens of the same species. So I wouldn't be surprised if you got another one and you had a different experience, but I also wouldn't be surprised if you had the same. As I see it though, while it can be frustrating at times, besides getting rid of the "holdfast" or base of algae missed by most herbivores, this scouring effect urchins have on substrates is an essential part of the carbonate cycle in reefs as they are pooping out carbonate and nutrient rich poop that benefits corals.
In addition to the research I linked to in my previous post here's a couple papers showing their importance in growing corals. This paper showed a significantly increased survival of settling coral larva and this paper showed a "3X" reduction of labor when urchins were used in coral propagation. FWIW, I wouldn't keep a marine system with out urchins regardless the negatives in their behavior.
