"what's on my fish" ?

Skin Fluke (Gyrodactylus),

A common type of freshwater parasite that can infect all types of pond fish are called flukes, which include skin flukes and gill flukes.

Both types of fluke are visually similar, with both species attaching themselves to the body of the fish to feed on upper layers of skin and gill tissue.

Koi can become infected with both types of flukes at the same time, but it is more common for just a single type to start causing problems unless water conditions are particularly bad.

Since flukes are microscopic, the only way to determine if your koi are infected is to take a skin/gill scrape and check under a microscope, or to try to identify common behavioural and appearance changes caused by the parasites. Like many other types of parasites, flukes are specific to different types of fish, and koi will have a particular fluke that specializes in carp.

Flukes can attach themselves to other fish for a short time, but they would

eventually need to find the correct host to survive and breed. Luckily, flukes are more of a nuisance to koi in the early stages and should not cause any long-term problems with health if treated quickly.

Flukes by themselves will irritate the fish, and will gradually stress the koi leading to them eating less and acting strangely.

As the number of flukes grows, koi will become weaker and the flukes will start spreading to other fish, so it’s always good to treat any possible fluke outbreak early to stop the problem from escalating

Skin Flukes (Gyrodactylus)

skin flukes attach themselves to koi and feed from the upper layers of tissue and membrane.

Skin flukes, however, do not attach themselves within the gills but instead will feed on the body of the fish, so can be found anywhere on the outer scales.

Skin flukes are smaller than gill flukes, around 0.4mm in length but can still be seen under a microscope after performing a skin scape of the fish. Visually, they lack the dark spots present in gill flukes but have a similar cylinder-shaped body and hooked opisthaptor on the tail for attaching to the host.

Skin flukes breed without laying eggs and give birth to live larvae, making new flukes immediately able to search for a new host fish. They show similar symptoms in koi and should be treated the same as gill flukes before they can spread.

Skin fluke symptoms include:

Rubbing against pond liner/objects

Irritated skin or ulcers forming (wounds)

Dulling in colour


Koi Not taking food

Since flukes and most other freshwater parasites are microscopic, the only way to “see” them and confirm they’re the problem is with a scientific microscope.

You would need to take skin or gill scrapes from your koi and then check the samples under the magnifying lens.

This would allow you to see both gill and skin flukes, but as it requires the use of professional equipment, it may not be an option for everyone.

What do they look like under a microscope?

If you don’t want to invest in a microscope to identify your potential parasite problem, you would simply need to look for signs of parasites from the behaviour and appearance your fish take.

Changes in colour, pattern, and behaviour are all signs that could point to parasites, with a precautionary treatment being advised to be on the safe side

Can flukes survive in very cold water?

One of the most common times for koi to get flukes is during winter, as their immune systems have slowed down to a crawl, making them more vulnerable.

Flukes can survive in both very warm and cold pond water, with the only change being their offspring’s incubation (hatching) period. Eggs laid by gill flukes will hatch within 4 days in the water around 20°c (summer) but can take 5-6 months to hatch in the water around 1-3°c (winter).

This means flukes that infect koi in winter lay their eggs and remain dormant throughout the season, and then start to hatch in huge numbers in spring. Even if the flukes themselves die off, their eggs will remain and start to spread when water temperatures rise.

For this very reason, it is good practice for koi keepers to treat their pond water for both bacterial infections and parasites just before Winter and then again at the start of spring.

This ensures koi are not infected during hibernation and provides the safest kick-start to the new year when Spring arrives.