I have a spiny sea urchin. I got him when he was tiny, but he tripled size in about two months! Finally I had to move him into the fuge because he was eating every bit of coralline in my tank (not to mention starving my algae grazers). I think it depends on the type you get. I never had the problem of him moving my rocks around. I would say a tuxedo might be a better choice.