Metronidazole should work. I wouldn't use it without Seachem's Focus, however, if you're feeding it. Focus is a binder that will cause to medication to stick to the food. Without it, you'll lose a lot of it into the water column.
If you don't see any improvement in 2-3 days with metronidazole, I would try Seachem's Kanaplex with focus. Metronidazole does have antibacterial properties, but is not an antibiotic; it's an anti-infective drug. Kanaplex is an antibiotic that is also easily absorbed through he gills, making whatever does come off the food more beneficial to the fish. The metronidazole isn't as harsh, so I would start with that one.
Your best bet is quarantine, but medicated food should do the trick. Just use a 1:1 ratio for the Focus and medication.