Koi Parasite Identification

If you think you may have a koi parasite problem because your koi or goldfish are flicking or scratching on objects in the pond or have a grey mucus layer on their skin then skin scrapes will need only be taken from two or three koi, if parasites are found, it is safe to assume all koi are infested by the parasite and therefore the entire pond will require treatment.

Guide To Identifying Koi Parasites and Methods of Treatment

Identifying Parasites Under The Microscope

Most koi parasites are not visible to the naked eye. In order to correctly identify any koi parasites you will need a microscope with built in light and a magnification of up to 400 times.
These are simple to use and are an essential piece of kit for any koi keeper.

To accurately detect any koi parasite causing an infestation of your koi, you will need to take a skin scrape. This involves using a microscope slide to carefully remove a small quantity of mucus from one of your koi. The mucus is then sandwiched between the slide and a cover slip and inspected through the microscope. We would recommend that upon purchasing your microscope, you ask your koi dealer to demonstrate this technique to you. This will help avoid causing any possible damage to your koi.

If you suspect you have a koi parasite problem, skin scrapes need only be taken from two or three koi, if parasites are found, it is safe to assume all koi are infested by the parasite and therefore the entire pond will require treatment.

Before any parasite treatment is used in your koi pond, it is vital that you know the exact volume of water and temperature, and that adequate filtration and aeration is provided. Parasite treatments will lower the dissolved oxygen levels in the pond so we recommend adding additional aeration when using any chemicals in your koi pond. You must also switch off any ultra violet sterilisers / clarifiers and ozone equipment.

Ichthyophthirius multifiliis (white spot)

Classed as a large protozoan parasite, white spot must initially be detected through the use of a microscope, however if left, its effects becomes identifiable with the naked eye. Koi infected by white spot appear to be covered in white spots the size of salt grains, where the parasite has burrowed through the outer skin layer


To multiply the adult parasite leaves the host koi, it then forms a capsule around itself. Within the capsule it divides and multiplies producing up to 1000 tomites (babies). These are then released as free swimming parasites to go in search of a new host koi.

This reproductive cycle is rapid and so early detection of this koi parasite is essential.

To effectively eradicate this koi parasite we recommend using a treatment which is a mix of formaldehyde and malachite green. Pre-mix solutions are available, but it is better to mix your own as the dose rate of formaldehyde should be altered dependant upon pond water temperature.

We would use Malachite green solution at a dose rate of:
12.5ml per 220 gallons.

Formaldehyde (36% solution) should be used at:
<54 ° F 15ml per 220 gallons
55-64 ° F 20ml per 220 gallons
>64 ° F 25ml per 220gallons.

A second dose should be applied five days later, and you should take further skin scrapes
following treatment to ensure the White spot has been eradicated.

Ichthyobodo necator (Costia)

This koi parasite is extreamly small (10-20 microns long) and a magnification of 100 times is the absolute minimum required to identify costia, 400 times is ideal. When taking a skin scrape and looking for Costia, try to avoid using a thick layer of mucus and add a drop or two of water to the slide. Costia is a very fast moving parasite and looks in shape similar to a comma, search the entire slide carefully but you will often find it easiest to identify this parasite along the edge of the mucus layer where this meets water.

costia parasite costia parasites costia on koi

Costia affects both the skin and gills of koi, and left undetected will multiply at an alarming rate and can soon cause mortalities of koi.
Symptoms of infection include excess mucus production, respiratory distress, and general debilitation, the eyes of your koi may also appear sunken.

The koi parasite Costia has a very simple life cycle, using a process known as binary fission, meaning each parasite divides into two.

Costia infestations can be treated using a mixture of Malachite Green and Formaldehyde at the same dose rate as described above for white spot. Another effective treatment is potassium permanganate.

Chilodonella cyprini.

Chilodonella is another protozoan parasite which effects the skin and gills of koi.
This koi parasite is typically between 40 and 70 microns in length and is oval in shape. Again, identification of this parasite can only be achieved through the use of a microscope.

chilodonella chilodonella parasite chilonodella koi parasite

The parasite multiplies quickly through division, although from time to time a pairing of two individuals takes place where some genetic material is exchanged, this process is known as conjugation.

As this koi parasite is an efficient swimmer, infection from koi to koi can also spread quickly, especially in overstocked ponds.

Effective treatment of this koi parasite can again be achieved through the use of Malachite green and formaldehyde mixed together, at the same dose rate as for White spot and Costia. In many cases, one dose is enough, however further skin scrapes should always be taken and secondary doses applied if required.


Trichodina is another small protozoan parasite which is commonly found on koi. Microscope identification is necessary, as it has an average size of 0.07mm in diameter.

trichodena parasite trichodena koi parasite trichodena fish parasite

This parasite is circular in appearance, and is often seen spinning and moving very quickly when viewed through the microscope. The parasite attaches to the koi using tiny hooks and holding discs.
The koi parasite Trichodina can quickly cause severe damage to the skin of a koi (as can all parasites) leaving the koi open to secondary infection from bacteria such as Aeromonas.
Trichodina can swim well and therefore this koi parasite can quickly infest an entire pond of koi, especially when the stocking level of koi is high. Trichodina multiplies by division.

Trichodina can be treated using the following:

Potassium permanganate crystals (BP grade) at a dose rate of:
1.5g per 1000 litres..

A second dose should be applied dependent
upon water temperature:
>68 ° F after 7 days
60-68 ° F after 9 days
<60 ° F after 12 days.

WARNING!! Always increase aeration of your koi pond as this chemical will significantly reduce oxygen levels in the water.

Gyrodactylus (skin fluke)

Gyrodactylus is one of two common worm type parasites which the koi keeper may encounter. The parasite is worm like in shape and it has hooks with which the parasite attaches itself to the koi.

gyrodactylus koi parasite gyrodactylus parasite gyrodactylus Parasites

This koi parasite reproduces live young. Being hermaphrodites, all of the adults are capable of producing young, and each one will carry a single larval parasite in its abdomen. Further more, this unborn parasite is also developing a larval parasite in its abdomen before they are even born, and in as little as one day after being born, those young can also give birth. So it’s easy to see that this koi parasite is very prolific, and one individual is capable of reproducing into thousands in a short period of time. When viewing this parasite under a microscope you can often see 3 or 4 developing parasites within each other.

If you carefully look at the photo below you will be able to see an unborn parasite.
Once this parasite is attached to a host koi, it lives and feeds on the mucus skin and blood of the koi. The parasite is capable of surviving without a host koi for five days.

This koi parasite can be treated using Formaldehyde and Malachite Green as described above.
Other effective treatments are Superverm, Flubenol and Masoten.

Dactylogyrus (gill fluke)

This koi parasite is very similar to Gyrodactylus in appearance. It has a set of hooks with which to attach itself to the host koi and these are surrounded by a number of smaller hooks
.dactylogyrus koi parasite gill fluke dactylogyrus koi gill fluke gill flukes on koi

The two parasites differ however in their method of reproduction, this koi parasite is an egg layer, and can lay up to two- dozen eggs per hour. Water temperature is important as the reproductive rate increases in warmer water, and decreases in colder water. The same applies to the time required for the eggs to hatch. In warmer water hatching can take only four days, whilst in colder water it may take as long as thirty days. This is a very important fact to remember when treating this koi parasite as most treatments will not kill the eggs, and they can hatch even after treatment and re-infect the fish. For this reason either the initial treatment has to stay active for at least four days, or you must do a second dose of the treatment.

To treat koi infested with this parasite you can again use a Mixture of Formaldehyde and Malachite Green, apply the second dose after five days and take further skin scrapes to ensure all parasites have been eradicated.

Other treatments include Superverm Flubenol and Masoten.

Lernea (Anchor Worm).

This koi parasite is most commonly found on newly imported koi, and should be dealt with by your koi dealer, it is rarely a problem for the koi hobbyist.

anchor worm on koi anchor worm lernia anchor worm parasite

It is visible to the naked eye and the adult parasite may reach 12mm in length, with anchor like appendages at the head. It attaches to the koi and the anchor penetrates under the scale and into the muscle of the fish, where it feeds. This parasite reproduces by laying eggs, two egg sacs are produced at the end of the females body, the larvae hatch from these egg sacs and swim freely until they come into contact with a koi, they then commence the cycle again.

The parasite can cause serious damage to the koi where it penetrates the tissue. These wounds sometimes heal very slowly and if untreated become infected with bacteria and fungus, it is these secondary infections that cause the most risk to the koi.
To treat Anchor Worm you must treat the pond to sterilize the adults, this treatment should be repeated after seven days to ensure any eggs that were unhatched at the first treatment are now sterilized after hatching. Now all reproduction has been stopped. The treatment used is Dimilin.

Now the adults must all be removed from the koi. To do this you will need to sedate each koi individually and carefully remove the parasite with tweezers, making sure you remove the entire thing including the anchor part. Each entry wound should then topically treated. Check every koi in the pond to ensure none are missed.

If you have difficulty removing the Anchor worm, try dipping the ends of your tweezers in a potassium permanganate solution, this causes the parasite to release its grip.

Argulus (fish louse)

Argulus is a type of crustacean, the parasite attaches to koi using suckers. The parasite then punctures the skin of the koi and feeds on blood and body fluids. It also releases a toxin when it punctures the koi, this toxin may cause erratic behavior of the koi as the effects cause intense irritation. The punctured areas of the koi will also be open to secondary bacterial infection.

argulus fish lice argulus koi parasite argulus on goldfish

This parasite can be introduced to the koi pond by visiting frogs and toads.

Argulus reproduce by laying eggs. After mating the female detaches from the host koi to deposit her fertilized eggs on the pond walls or any other surface. Upto 500 eggs may be laid. Dependent on water temperature the eggs take between two and four weeks to hatch. The newly hatched parasites will swim to find a host koi. They will reach sexual maturity after 20 to 50 days depending again on the water temperature of the pond, then the cycle starts again.

To treat an Argulus outbreak you will need to sedate and inspect every koi in the pond, removing all adult parasites with tweezers. The pond must then be treated to kill off the juveniles. This treatment should be repeated two or three times at intervals of seven to ten days.
The most effective treatment is Masoten.