Do you have a test kit to check the water ammonia, nitrite and nitrate?
Having the water be in tip top parameters /safe level is the best way for fish immune system to be able to deal with the n=medical problems /healing.
When was the last time you have done a water change and how much did you change out of the 125G?
If it was around a 25% pwc
with in the last week or 10 days, I would recommend doing 2 back to back water changes of 30% change 30%, then change another 30%.
It makes the difference in the new and old tank water less stressful on the fish.
If the water change has been longer 2-3 weeks, then I would do a 2 - 20% back to back water changes. Wait at least 8-10 hours and then do another pwc
maybe a 25%.
If it has been a few months then do 10% back to back, then another 8 hours same thing, and then repeat a 3rd time.
Having the numbers of the parameters are needed to make sure everything is in the safe level.
Also important is to check if it could be a burn from a heater. Maybe a spot it could get stuck?
A wound can certainly become infected and cause this type of injury, as could some other type of serious parasite infestation.
Do any other fish seem to have any injuries, red spots, worms around the eyes, gills, body, anal vent? They could be super tiny. Are they flashing/ rubbing against items in the aquarium?