I thought about the same stuff (including treatment) but I am always cautious to do that since the stress of the capture and move may be worse that the issue itself. In the +20 years I've been keeping fish/coral I have never seen anything like this so I 1st want to understand what it might (or is) be.
The fin is nor eroded away, only the very top if the fin appears affected (see image 2). The fish is an active swimmer, swimming correct and very strong, is able to move the fin (with no suggestion of detachment, no listing to it's side, etc). The tank itself can be seen on my webpage; there are many SPS and soft corals (no anemones). My 1st thought was that maybe the fish scratched itself on an SPS. Tank mates are peaceful; the most aggressive is a Clown but he does not appear to be bothering anyone. Other fish are also quite peaceful; I say and watched the tank for a very long time last evening just to be sure all critters where playing well; and they where. The fish in question is eating like a pig (no loss of apetite).
In short, I have not found anything wrong with the fish except for the marks. Something else about the marks; they appeared overnight; it was fine at lights out 2 evenings ago and this appeared the next day; that is what lead me to feel it is a hard scratch or a fight (which I could find no evidence of; aka no fighting).
Tank critters, Hectors Goby, Clown, Snowflake Eel, Blue Tang, Coral Banded, Cleaner Shrimp, Fairy Wrasse, Dragonnet, 2 FireFish, 2 Spotted Cardinals, Marine Beta, Sailfin Tang. SPS, LPS, softies, Clam, Fire Scallop.