I have two banggai cardinals that have been my pets for two years. They are tank raised and I originally purchased 4 pretty small fish, really small actually. Two were decimated after a few months. I assume the existing two are a male female pair. Recently one of them is having a color shift. The fish seems fat and healthy as ever. Over a week or two the back half of the fish has turned almost solid black. I’ve been keeping saltwater tanks since high school, I’m currently 56. I have never kept banggais before. Is this normal?