Potassium Permanganate is a useful fish disease treatment:

Protooan Parasites such as Trichodina, Costia and Chilodonella.

Bacterial Disease: Gill Disease, Open Sores.

Fungus.

Reduces Organic Matter to improve water quality.

It will also oxidize dissolved organic matter, reducing the biological oxygen demand and improving water quality and clarity. 

Disinfect your plants before you put it in your pond.

Seek Professional help before using Potassium Permagenate. This product is dangerous and can kill your koi if not used properly.

Dr. Johnson, Vicki Vaughn are excellent sources.