Identifying Parasites on Koi Fish

An important aspect of good koi husbandry is being able to identify when our koi are sick and how to treat them without performing any shotgun treatment.  Koi fish are an investment and like any investment the more knowledge we have the better we can care for that investment, and get long-term results. 

Below is an illustration of common koi ailments and parasites.

Koi Parasites

Symptoms of Koi with Parasites

Parasites are not visible on koi; there may be some indication that koi have parasites when demonstrating the following:

  • Loss of appetite and lethargy
  • Clamped fins
  • Rubbing and flashing
  • Ulcers or lesions, redness on the body, and red streaks on fins
  • Foggy eyes
  • Unusual gill activity and hanging out more by high oxygen areas, such as a waterfall.
  • Skin has grey-white opaqueness, and color discoloration.
  • Lying on its side at the bottom of the pond
  • Isolation from other koi

How to Check for Parasites

Observing any of the above is an indication that your koi may have a parasite, now how do we know for sure?  The only way to check the koi for parasites is by taking either a gill sample or scraping some mucus off the body of the koi and inspecting those samples with a microscope. 

Below are some things you will need to identify the parasites:

  • Microscope (magnification up to 400x to 1000x), slides, and slips, optional a camera that attaches to the scope so you can save photos and videos on your computer of parasites.
  • Tweezers, swabs, snipping scissors for gill snips.
  • Sedative for koi, like clove oil (Eugenol oil) or MS222.
  • 2 tubs for koi, 1 to sedate, 1 to recover after sample removed.
  • Air pump with air stone to agitate water and provide oxygen.
  • Koi ID book to identify any parasites found, we use “Koi Health & Disease 2” by Dr. Erik L. Johnson

Now that you have the tools needed, if any koi exhibit any symptoms from above you can quickly begin the investigative process.  It is recommended to sedate the koi before attempting to take any samples, once the koi rolls on its side it is easier to handle and scrape the area for scoping. 

There are 4 recommended areas to scrape for parasites:

  1. Under chin and out to pectoral fin
  2. Scraping any obvious wound
  3. Flank of fish
  4. Base of tail out into caudal fin

Koi Parasites – Stylised Diagrams

Parasites ID images

Treating Koi with Parasites

All these parasites when found, can be treated in the koi pond, the earlier the detection the better.  Each parasite has a different method of treatment, which uses different medications to eliminate it.v

Below is a chart with the most dangerous parasites found on koi and the recommended treatment.

  CostiaFormalin based (Proform C)**  or Praziquental
  TrichodinaPotassium Permanganate
  ChilodonellaPotassium Permanganate or Proform C**
  IchProform C**
**Make sure no salt is detected in pond as Proform C is not recommended in use with salt

Costia and Chilodonella

The most dangerous of the above parasites is Costia and Chilodonella as both can kill off a large number of koi in a short period of time if not caught early enough.  Both of these parasites survive in all temperatures including water temperatures as low as 45° so when treating them use the appropriate temperature-based medication.


Flukes can kill off large numbers of koi but usually in a slower process, as they need a large number of Flukes to attack the koi.  Once the Flukes attach to the koi they will stress the koi out, causing the koi to flash (rub up) against the wall of the pond or any object in the pond.  This will cause skin irritation or sores.  Because the fish is now stressed from the Fluke it will stop or decrease eating and may become more isolated. 

There are 2 types of Flukes, Skin or Gill.  Gill Flukes are found in the koi’s gills, and when mass produced can kill the gill area causing the koi to succumb due to not being able to breathe.  Gill Flukes can hatch eggs at the rate of 20 eggs per hour, in a water temperature of 75°.  Skin Flukes attach to the body of the koi, they do NOT lay eggs, but instead give live birth and is continuous, it takes 5 days from egg to birth.  Fluke-infested koi will be thin and emaciated, may develop a milky film on its body, they will have skin irritations (red areas) when the Fluke have attached and been feeding off of.

Preparing yourself with the right tools and knowledge will help in diagnosing any koi parasites allowing you the opportunity to keep your koi healthy and happy.