I bought 20 of them 2 weeks ago. So far, I haven't seen them harm any corals. I watch them but haven't really even seen them eat anything yet, not even hair/bryopsis algae. However, I saw a large one beat up a similar sized scarlet crab and take its shell. I have photos of this fight that I'm going to post. He didn't kill the scarlet, he just took the shell.
I bought them because I read that "This small crab is the best algae eating hermit crab we have tested" and "These red legged crabs are very effective at eating hair algae."
I'm not really sure who's quote these are but I found it from here:
http://www.athiel.com/lib4/digueti.htm
[ 29 April 2002, 13:40: Message edited by: Sam W ]