Likely mechanical injury and then bacterial infection.
Have you cycled your QT before?
Do you have UV?
Not so much for this fish, in the future, in order to treat fish in a QT often one needs a cycled tank; otherwise, frequent water change will be necessary to rid the QT of ammonia.
In this case, antibiotic treatment is likely called for. If a fish is already infected then UV is no longer called for, as UV degrades antibiotics.
UV is useful in prevention; that said, if there was a large open wound from injury, then UV may not do much to prevent infection.
Some antibiotics (chloromycetin for example) will destroy nitrification bacteria; some will only (neomycin kanamycin) depress it somewhat.
