Arrow Crabs are typically inverts to be left out of home aquariums, or kept as the sole inhabitant. First off, some have been known to eat coral polyps, most eat BristleWorms, and all I have seen, try to eat any and all fish in the tank. If you read the profile for them here on SWF.com, it mentions their natural propensity towards live prey. They typically scavenge from the LR and the sandbed for BristleWorms and the like. So, in their scavenging, they will eat any soft corals they come across. As for eating fish, yes, I have had TOO MUCH experience with it. I had an Arrow Crab that ate my Engineer Goby, before I had realized their natural tendencies, and just didn't notice it missing, and then lost my Volitan Lionfish right before my eyes. The Arrow Crab plunged its claws into it in one foul swoop
as the Lion was "slalking" some swaying caulpera. The Lion never had a chance. I tried my hardest to keep that Lion alive, but it didn't make it. Needless to say, the Arrow Crab was traded back to the LFS in two seconds flat.