You could have a crab or a mantis shrimp and don't know it. Its happened to me before, in my forty gallon I had a gorilla crab and no fish went to sleep near his rock, and all the new fish I put in died because of him. In a new tank I bought live rock and I received a free mantis shrimp in my rock, woke up one day to see four fish dead and to see a mantis shrimp running around. Hope you find out what did it.