have you checked for pests when the lights are out with a flashlight you usually see them better at night. What kind of fish do you have any blennys or gobys that can be perching or nipping
I have 4 fish 2 gobys , clown and anthiud. I just perched him up there he was usually in the sandbed. I thought sandworm might be the prob so I moved him.
gobys and blennys have been known to nip at mantles i had a clown goby taking chunks out of a squamosa and a algae blenny nipping at a maxi awhile back and my buddys comb tooth blenny is doing it now in his tank
Aisol blues w 30 b 80 rb 80 10 in awl .I don't think it was the lights he was over a year old .yes I think they were chowing down on him. Pisses me off here is a pic of him 2 weeks ago.