I got one several months ago because I was going to run a hypo treatment on my tanks at work which would've killed the purple lobster. So I adopted him. About 2 days after I put him in my tank I found his shell and felt sorry because he'd died. We put a snowflake eel into the tank just a few days ago and suddenly there's the purple lobster!!!! So nope, he didn't seem to harm a single thing in my tank (fish, mushrooms, leather coral, hermies, peppermint shrimp, turbos) but he hid so well I didn't even know he was there for months!!! On the other hand, I'm sure he was doing a good job of cleaning the tank when we weren't looking, he had to be eating SOMETHING all that time, so you could consider him a part of the cleanup crew I s'pose.