Medications
Your Koi should only be deployed after a preliminary diagnosis is derived. The starting point for the diagnosis is with your water quality. THEN you can spend time and energy diagnosing diseases per se and deploying treatments.
You might like this article on dosing Potassium permanganate in smaller systems.
I've also prepared a "Shotgun Remedy" for your study and consideration.
You might also benefit from our discussion of bacterial infections.