I have two in my reef tank, a pincushion urchin and a black long spine. Neither of them have bothered my corals, however, my black urchin will go eat any dying matter in the tank, including corals. He will frequently eat a bit of dying xenia (I constantly have to groom that and I just drop what I pull off into the corner). I haven't noticed any difference in my coraline growth. The black spine is among my most effective animals in the clean up crew. Of course, the big down side is that he has venomous spines, so I have to continually look out for him. His spines are 8" long now (when I bought him 6 months ago they were about 2"), so it seems like he is constantly in the way. I've been stung a few times, no fun, but not really that big of a deal either. The pincushion doesn't sting and its spines aren't that sharp so I can pick him up and move him around. Overall, I highly recommend urchins.