I am assuming this fish was not quarantined at all?
How was it acclimated (detailed please)?
Are we talking about Valenciennea strigata?
Pimafix and Melafix are antibacterial remedies but will only benefit to some degree. What exact problems they will treat is even unknown to the manufacturer. It is a very hit and miss remedy that I personally would not trust the outcome of a "maybe".
The two main causes of what your seeing are either a systemic bacterial infection or a low pH problem. If the 8.2 pH is accurate and consistent (day/night?), I would look towards the systemic infection. In which case a QT stint with Maracyn II for a week would be in order. If this is very mild, you can also try several very large (25%+) water changes over several days this week. It could help the fish help itself in some cases, especially if this might be an external laceration. Given the presence of the pistol, that is also a possibility. If the problem persists, treating in a QT is the best option. It need not be pretty or even a tank, just functional.