Well, lesson learned. Do not keep more than one Chromis. They will slowly bully each other until only one is left. Had read in here this behavior could happen but they looked so pretty swimming together I chose to ignore the warning and had gotten 3 green Chromis and 2 blue Chromis. Now I only have 1 green and 1 blue left. They don't seem to bother each other (so far), only the same kind.