It's a bacterial infection. To treat these types of infection you isolate the fish in a bare tank, which you have done, and add anti-biotics, also done.
Kanaplex contains Kanamycin, which is a type of anti-biotic. There are other anti-biotics too including Tetracycline, Doxycycline, Erythromycin, etc. Different anti-biotics work on different bacteria and without knowing which bacteria is causing the problem it is impossible to say which anti-biotic to use. It's basically add medication and hope it works.
The most effective way to use anti-biotics on fish is in a bare container. You add a full dose of medication and aerate the container. 24 hours later you wipe the inside of the container down with a clean fish sponge, change all the water and retreat the container with another full dose or medication.
You continue doing this for at least 1 week but no longer than 2 weeks. If there is no improvement after 48 hours then the medication is probably not going to work and you should try a different type.
If the fish is worth a lot of money or you are friends with a vet, you can get the fish checked by a vet and they can take a swab of the infected area and look at all the different nasties crawling around on it. They will probably give the fish an injection of anti-biotics and send you home with some more anti-biotics and a big bill.
If you can get the medication into the fish (via food) it usually works faster and more effectively. However, if the fish is not eating then that doesn't help.
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Most bacterial and protozoan infections are caused by lack of tank maintenance (water changes, gravel cleaning and filter cleaning). Big predatory fishes like Oscars produce huge amounts of waste and regular gravel & filter cleaning is essential, as is doing big (75-90%) water changes at least once a week.
