I have had a small urchin for about six months. Its got short purple spines and its inside is white. It eats algae and when I have had a Chromi die, it ate that too. I accept that it knocks down one of my frags every week. But recently several of my zoanthids and palythoas have suddenly sustained mortal injuries. Everything else is healthy. My favorite palythoa is a stump! My tank has nearly no algae, except for the glass so maybe its getting too hungy? Its pretty much all corraline on the rocks. Could that little jerk be eating my coral?? Everything I read was positive about them.