When I first introduced my maroons, there was some fin nips for sure so this may be the case with yours, but I also had a gorilla crab. I noticed large sores on the sides of a couple of fish only to have the injured fish disappear the next night before I figured out what was going on. I did the whole trap thing, and it work surprisingly well. I put a shrimp tied to a piece of rubble in a plastic water bottle that had the end cut off and put it near the crab rock. Waited patiently with a red filtered flashlight and a net in the tank until about 12:30am and saw the crab venture out towards the bottle. I couldn't wait any longer so I shoved him the rest of the way into the bottle with the net. Gave him away to a mantis owner!
