Has the fish been fighting with any other fish in the tank? Is the wound large or small? Clean looking or nasty? Finally, is the wound on the fishes lateral line?
In
most cases, a small clean wound will heal on it's own as long as the water quality is good. Even though you have 0 readings on ammonia/nitrite/nitrate, I would still do a water change....there are so many things you can't test for in your aquarium and a water change can really help.
If the wound is on the fishes lateral line, then it may be the start of HLLE (head and lateral line erosion). There are many different thoughts on what causes this.....poor nutrition, poor water quality, stray electrial voltage, running carbon, stress, etc.....again, the best thing to do is a water change, and make sure you are offering the tang the correct foods like nori.
If you think the wound is caused by fighting, then of course you will have to remove one of the fish......
And if the wound is very large, deep and nasty, then you have other problems.....
and you will need a hospital tank to treat the fish....