I have always had a ton of them (I have a problem feeding to much) and they have never caused me any problems. There are definately some bad ones out there that eat corals, especially zoas but I have never introduced any. Mine are all the descendants of hitchhikers on my original live rock in 2006 and I consider them a valuable part of my CUC and find them interesting. The thing I like best about my tank is the opportunity to expand the diversity of sea life that is possible to keep in saltwater and what better than tiny starfish that won't outgrow the tank and eat the algae and other things that make the tank dirty.