crab good or bad ?

Can anyone tell me if a crab, dont know where it came from or what it is, will
prey on the fish ? I added seven blue reef chromises a few days ago and now can only find four. This crab is maybe 2.5 - 3 inches, 1/2 in. claws and is a dark color, he stays under the rocks. Also found what was left of one brc, some bones, dont know if he killed it or ate it after it died. I think I know the answer to my question - get the crab out, if thats the case, how do I get it out? probably a real pain in the a**
Thanks, Will

 

Chromis are interesting. If you have too many in a school, they often beat each other up randomly. If you have too few in a school, they usually gang up on one or two and kill them.

I'm sorry for your losses. I truly don't know if they are killing each other or if the crab did it. However, I will say this. If I had a crab you are describing in my reef tank, I would have it living in my sump within 24 hours even if it meant staying up all night to capture it. If it was grey with red eyes like an Eriphia sebana, I would tear my entire tank apart to get it into the sump before 24 hours. I'm not a fan of other Xanthid crabs either.

Your crab is too big for most of the common traps. Maybe you could use the tipped glass method. You put some meaty seafood in the bottom of a glass and then lean it against you LR. The crab will go in overnight and won't be able to get enough traction on the glass to get back out after eating.

 

Yeah, the chromises do chase each other, seems more playful than aggressive. there is a pair of clown fish in there and theyre not happy with the chromises so who knows. Ill try the glass for the crab but it doesnt stray too far from that rock
thanks, wil