I had one in a 20 reef I did and like mentioned before it was an opportunistic eater. It personally liked the leathers that were in there. To bad it didn't have a taste for xenia's. At the time those things were even growing on the overflow box and needed to be taken care of anyway. However, emerald crabs IMHO
are about 50/50. You might get one thats okay like your first but then you might go threw a couple of them that arn't. Personally I won't put them in the next reef tank I do. Probably only use them in FOWLR
setups from now on.