If the fin around the rays is missing it definitely sounds like biting. My guy is a chronic tailbiter. I did everything I could to try to destress him. Then I thought maybe it was boredom and did everything I could to keep him from being bored. I tried a smaller tank, tried a bigger tank, rearranged his decor, changed his decor, left him alone with no light and towels wrapping his tank for several days, turned off the filter, turned on the filter, got a new filter, fed garlic-soaked pellets in case he had parasites, let him see my female for a few minutes so he could flare, floated a ping pong ball in his tank...nothing helped. It's so frustrating!
He seems to never stop biting. For now I'm just keeping on top of regular water changes (with an additional one per week) to prevent infection, he'd be in a salt treatment permanently otherwise, because he just doesn't stop! But I'm definitely afraid of infection/fin rot since he always
has exposed, injured fins.