Just thought I would throw this out there, I got one of the purple ones that Melosu showed you and it was in my 40 gallon with my 1 small sebae clownfish and lawnmower blenny. The lobster would always go into what seemed like attack or defensive mode with his claws out whenever the clownfish come by. I only had him for one day but he tried multiple times to try and pinch the clown.
I returned him right away just because if he was gonna be a problem and nip my fish's fins then I didn't want him. Also, he never got the clown because he was too quick but he was definitely trying and who knows what would have eventually happened. I would recommend against them :T, im gonna use my store credit and get an ACTUAL peaceful shrimp instead of trusting the LFS