Based on my experience with a butterflyfish and the same looking infection (very common area as it's the widest point of the fish where lots of water passes by making for a thinner protective coating and more prone to infection), Kanamycin didn't have any effect after a few days, so I switched to Nitrofuracin Green Powder and it worked wonderfully. Then I put the fish back in the main to continue to heal in a more natural environment. He's doing great to this day, but still has minor infection outbreaks from time to time, they look like little pimples. When I see this I soak his frozen food blend in Selcon and they heal right up.