Not positive. Im leaning towards an over aggressive aurastus. She had her fins slightly messed up that morning and I did see one time the aurastus go after her pretty hard. Once I'd seen that, I'd quarantined her. Several hours later she went. The strange thing though is that she didn't look in bad shape other than lying on her side. She swam fine for awhile. All the other fish in the tank are fine. The aurastus in question, while aggressive, never goes after any one of its tank mates. There are currently 25-30 fish in the tank ranging in size from fry to 7 inches and all are doing incredibly well.