These things can be so hard to diagnose because you don't know if the wound is from another fish or bacterial. And even if you determine bacterial, it may be a secondary infection, not the primary cause of her problems. Wish I had more to offer. I think if she improves once you isolate, you'll have your answer, aggression. If not, then almost certainly disease or age.She has a wound on the back of her side. Small, bite-sized.