It is very easy and not expensive to order kanamycine sulfate from Jehmco.

http://jehmco.com/html/medications.html

All you have to do is fill up a form that they send you to confirm you are not using it for resale etc..and it is not for commercial fishery and sign it and return it to them by fax for the first time order. After that they will ship you kanamycin and metronidazole for very affordable! You only have to fill the form once, never after that.

I bought a few times from them metro and kanamycin and very happy with this company.

I would not treat kanamycine in the main tank but if you can get the fish to eat that, they have medicated food as well.