Koi may or may not even NEED a treatment for fungal infections.
Fungal infections are caused by chilling, mostly. Poor water quality and a sagging pH can contribute. In ponds, fungal infections may come under control just with the warmth of spring. But if you want to treat, there are three medicines that work. Dosing is available on DrJohnson.com
Formalin Malachite will treat "everything" including fungal infections. It's more "caustic" than it needs to be considering that Methylene Blue will work on fungal infections without the impact of Formalin but sometimes, you want to blast the whole lot, of everything including fungus.
The above is the methylene blue powder. What you see at right is the methylene blue prep expressly for fish. The same company makes an 8 ounce bottle. Personally, I'd look at the Kordon product for safeties' sake.
There’s a warm water fungus that kills Koi, and kills them hard. I can tell you how to look at that. For one thing, it happens in warm water, not cold. The treatment for that fungus (once it’s diagnosed is to cool the water with partial water changes and then daily dosing with Eco Lab’s BSDT Formalin and Malachite Green.