What species of fish? Do you have a hospital tank available?
I would promptly remove the wounded/infected fish to a hospital tank and (depending on species) add 1-2 tbls of aquarium salt per 10 gallons and turn up the heat to the fishes higher tolerance range.
I'm partial to not automatically using antibiotics, I would try some Mela-fix and /or Pima-Fix first for a week (follow product directions carefully).
If the infection increases or does not respond then I'd use a antibiotic.
If your fish had tankmates (I see in your profile he probably does), I would advise raising the tank temp slightly and a light dose of salt as a precaution, healthy fish should be able to fight off most disease but if its some nasty like Columnaris you can't be too careful.