I've had a double head Duncan in my tank for about a week or two and it's been fine. Both heads open and such, last night I did my weekly water change as norm and they were both still there. It's the afternoon of the next day and when I look at my tank one is completely sunk in and gone? All that's left is one still open and the other is like it was eaten or something. Any thoughts? The only fish I have in my tank are two clowns, pistol/goby, Mandarin dragonet and a cleaner shrimp. I've seen some things on cleaners but mine hasn't learned about corals yet with bullying them, at least I think...