Parasites - Skin & Gill Flukes

There are two main categories of freshwater flukes; Dactylogyrus, gill fluke and Gyrodactylus, skin fluke. They feed on the epidermis of the skin and gills of the host koi fish. The fluke parasite on koi will thrive in poor pond conditions such as low dissolved oxygen, heavy organic matter load and pond overcrowding. Plus, flukes do quite well in cool water temperatures.

Both types of fluke parasites have a slender cylinder shaped body. On the bottom, or foot end, of Dactylogyrus and Gyrodactylus is a pair of large anchor hooks and a rim of tiny hooks called a haptor. 

This haptor acts like a suction to dig in and adhere to the host. These hooks cause considerable damage as the parasite moves around on the fish. To make matters worse, the haptor usually carries and transmits bacteria into the injured tissue whereby encouraging outbreaks of Aeromonas and Psudomonas (ulcers).   

As the obnoxious parasites move around the host they cause damage and irritation. The koi’s reaction to this assault of constant digging and chewing is excess mucus production.

Koi Fish Gill flukes

Dactylogyrus, the gill fluke parasite in koi, is recognized by 4 tiny eyespots on the head. It also has hooks on the foot that it uses to adhere to tender gill membranes. The parasite stretches out and feeds on mucus and filaments in the gills.  Heavy infestations mutilate the gill tissues causing them to become disfigured and build up scar tissue. Over time parasitic activity will interfere with breathing and gill function. Extensive damage to gill tissue results in reduced oxygen transfer to the fish’s blood supply, making the host koi becomes listless.  Dactylogyrus breed in the gills and deposit its eggs within the gill filaments. An adult gill fluke is capable of laying up to 20 eggs per hour in 75⁰F water. The eggs are then swept out into the pond water by the fish’s respiration; water flowing across the gills as it breathes. The eggs will hatch within a few days depending on the water temperature and the tiny larvae begin their hunt for a host immediately. Juvenile flukes can live for 3 days in free-swimming mode while searching for a suitable host. These offspring will become mature in 10 days and begin reproducing to create many more blood-sucking parasites.

Koi Fish Skin Fluke

Skin fluke parasite, Gyrodactylus, is physically similar to gill fluke; the body shape is the same and it also has a series of gruesome hooks called haptor for attaching to the koi. However, it has several differences. Gyrodactylus koi parasite is generally found on the body or flank rather than on and around the gills of a host fish. There are no eyespots on the skin fluke. The Gyrodactylus parasite gives live birth rather than laying eggs. The gestation process is continual; as one larva is delivered several more eggs move into the female’s uterus to develop. The maturing embryos are identifiable within the body cavity of the adult Gyrodactylus and larvae will emerge in approximately 5 days. Young fluke larvae get busy dining on the host koi immediately following emersion from the uterus.

Skin flukes may occasionally be found in the gill region of the host, and likewise, gill flukes might show up on the skin of the host fish. Regardless, both types of fluke parasites will adhere to an unsuspecting host to feed and breed, and will be detrimental to the fish’s health.

What are koi fluke symptoms?

The symptoms of fluke parasites on koi fish are many. Look for fluke in koi fish that exhibit these signs: lethargy, redness or irritated skin, excessive mucus covering on skin, isolation and weakness. You will notice that your fish become thin to emaciated, become listless, and see the appearance of a milky film surrounding the fish. This is referred to as ‘gray slime disease’ and is the koi’s response to the aggressive feeding by heavy infestations of the parasite. Scraping or rubbing in an attempt to rid the pests are also symptoms of fluke parasite on koi fish. Bacterial infections may follow advancing to ulcers and scars.

How long do fluke parasites live?

An adult fluke’s life span will range from two weeks to one month. However, in cold pond water the eggs, larvae, and the adults are capable of hibernating for up to 6 months.


Skin & Gill Flukes have various options of treatment.

• Kusuri Fluke - M
• Kusuri Fluke Solve 
• Kusuri Fluke - S 
• Pond Medic Plus Formalin 30% Treatment
• NT Labs Flukasol - M
 Chloramine T *Kusuri* 15-20 grams per 1000 gallons for 4 doses. Not effective in temps below 11̊C
Potassium Permanganate
• Potassium Permanganate (PP)

Pond treatment for parasites and snails: 2-4 g / 1000 liters.
Keep on adding in small doses of 2g/1000 litre to keep the pond water pink for 1-2 hours

Pond treatment for bacteria and ulcers: 2-4 g /1000 liter
Keep on adding in small doses 2g/1000 litre to keep the pond water pink for 8-10 hours

Dip:  very effective for parasites, bacteria, ulcerations etc. Use 1 gram per 10 liter water for 7 minutes. Always aerate and ALWAYS remove the fish if it gets restless and jumpy. Control timing precisely and do not treat too many fish at the same time unless you can time the treatment of each fish correctly.

This treatment tough, can push an ailing fish ‘over the edge’. Therefore fish already weakened by disease should first be treated with a 30-50% dose first time. Alternatively simply shorten you dipping to 1-2 minutes if the gills are infected and necrotic.

Important. Always de-clog the gills with a Peroxide (hydrogen peroxide) treatment to the pond. It not only terminate the action of the potassium permanganate, but will clear the water from the brown colour. Will also aid in removing the brown coloured necrotic tissue from the gills. 


I want to warn about use with salt. Salt (NaCl) is additive to potassium permanganate. To strip parasites from basically HEALTHY FISH can be tolerated, but sick, weakened fish should never be subjected to such treatment and will surely die. Always test salt level or ask the pond owner about his salt regimen before deciding on the protocol for the potassium permanganate treatment.

For certain phenotypes of the Doitsu koi the uptake of oxygen is poorer for certain physiological reasons. They stress more quickly and die due to oxygen starvation. Use half the recommended dose/time when dipping the first time at least to determine their suitability for the treatment..

Any treatments added are done so at your own risk. It is your responsibility to know your pond volume and to calculate dosages correctly. 

Golden Rules Of Treating Koi

Scrape the Koi first before treatment

Identify which Parasite is causing the problem before treatment

Make sure the pond water is salt-free because it can harm Koi when used with most chemicals

> Calculate the Volume of Water accurately

Turn off the UV Unit when treating your Pond

Test your Water Parameters before Treatment

> Follow the label’s instructions and do not overdose. Chemicals can cause serious damage to your Koi if amounts are miscalculated