I would echo what many others are saying about a crab or mantis shrimp as I had the same issue when setting up my tank a few months ago.
I had about 70lbs of liverock in a 90 gal tank and 5 chromis. All fish seemed healthy but every couple of nights a fish would disappear by morning. I could never find any of their bodies.
One morning I checked the tank and found a dead Gorrilla crab. I figured that it was the cause of all my problems but I kept losing fish after that. I ended up finding another 3 pretty large crabs after that.
So as others have said you could have alot of crabs in the amount of liverock you have as I had at least 4 with only 70lbs of liverock.
I didnt use a trap for the crabs but i did find that when i fed frozen brine shrimp, i could see crab claws coming out of the rock trying to get the food. I then was able to pull the live rock out and use tweezers and a screwdriver to push the crab out.
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