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.
Potassium permanganate
Salt
Chloramine‑T
SupaVerm
Diflubenzuron
Acriflavine
Formalin
Praziquantel
Fluke Tabs
Ulcer‑Aid Rx
More
You might also benefit from our discussion of bacterial infections.