Chilodonella

Chilodonella is microscopic. You won't be able to see them on your fish with the naked eye. Chilodonella can cause havock in a koi pond at any time of the year but is most prevalent in the spring, or cool water temperatures. Left untreated, Chilodonella can kill alot of koi in a small amount of time. You may notice fish acting lethargic, even laying on their side on the pond bottom, then quickly turn upright when startled. 

Treatment Recommendations 

ProForm C or Terminate are great products to use if your Koi have contracted Chilodonella.