It looks like the scales from that site have been removed, a pretty clean looking injury or wound, but the several month time period without healing is the real question,
IMO.
Initially I would've guessed a injury from hitting a object in the tank during a panicked dash or some such, but those usually heal fairly quickly.
My Oscar self inflicts this type of injury a couple times per month, but they heal within days.
Next thought is a injury caused by repeated rubbing against a object, hence the prolonged exsitence of the injury, rubbing or scratching because of a parasite or some such, or trying to hold itself in position against something because of excess current perhaps.
Or a wound caused by another fish or parasite, constant picking by other fish or parasite would prolong the healing.
Yet another thought is a burn from laying up against a heater.
Well, those are my possible explanations from what little info/pics I have to go with.
What are your suspicions?