There is currently quite a Costia outbreak and it has become difficult to eliminate it. The secret in  my opinion is to recognise it early. The one best telltale symptom of Costia infection is a white like mucus layer very visible on the hi of Kohaku koi on their heads where there ia no scales. Also a sandpaper roughness on the koi skin, although once you feel the roughness it is already quite late. If in doubt rather treat.

The two best treatment methods I have found is Acriflavine with salt at min of 4 Kg per 1000 Litres. 

Then I also like Malachite Green @.2 gram with Formalin @25ml per 1000 Lt. This I like to repeat in 5 days time.

I do see some do advise the use of PP but I dont think it will be effective.

I will be adding some personal thoughts on this opportunistic parasite during the next few days.