I had a baby midas die suddenly when I took away the fish he was beating on and left him alone in the tank. He just slept for two days and then just died. The best theory I could come up with is that he died of depression after I took his rank away. Cichlids of course can get depressed when you take them out of a dominant situation to solitary confinement, i have seen this many times. I dont know if this applies to your situation but its food for thought i guess