How to Treat Koi Gill Flukes (Fish Safe Treatments)

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 scrap 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 their own 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 cause irritation to fish, and will gradually stress the koi leading to them eating less and acting strangely. As the number of flukes grow, 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 in its tracks!

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Different Types of Fluke Parasites (Comparison)

1) Gill Flukes (Dactylogyrus)

Gill flukes are the type of fluke you more commonly hear about, as they’re easier to identify in both visual appearance and symptoms. They’re larger than skin flukes, measuring up to 3mm in length, but you will still need a microscope to properly identify them as they’re transparent and still too small for the naked eye.

Gill flukes have a slender cylinder body shape, with dark spots on the head of the body which skin flukes do not have. They also have a set of hooks known as an “opisthaptor”, which is a specialized organ for attaching itself to the tissue of fish. Both flukes have these hooks which are similar in shape, so the easiest way to identify the difference under a microscope is the dark spots present on the gill fluke.

Also, unlike skin flukes, as their name suggests, gill flukes attach themselves primarily to the gill tissue of fish rather than the body. Within the gill, they feed off mucus excretions and tissue and can cause open wounds, ulcers, and even infection from bacteria and fungus if they’re left untreated.

Common gill fluke symptoms include:

• Rubbing against pond liner/objects
• Constant “chewing” behaviour
• Irritated gills or ulcers forming (wounds)
• Dulling in color
• Hiding/shyness
• Koi not taking food

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2) Skin Flukes (Gyrodactylus)

Just like gill flukes, skins 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 outer 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.

Common skin fluke symptoms include:

• Rubbing against pond liner/objects
• Irritated skin or ulcers forming (wounds)
• Dulling in color
• Hiding/shyness
• Koi not taking food

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Flukes Under Microscope – What Do They Look Like?

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 can check out our guide to the best microscopes for assessing aquatic microorganisms in your pond here. 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? The below video shows an aquatic fluke under 400x magnification using a professional 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 color, pattern, and behaviour are all signs that could point to parasites, with a precautionary treatment being advised to be on the safe side!

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Where Do Koi Fluke Parasites Come From? 

Flukes, just like any other parasite, could have always been present in your pond, but it’s more likely they were brought to your system by visiting animals and wildlife. Both predators and pests can bring all sorts of harmful bacteria, fungi, and parasites to a fish pond unless you implement deterrents.

Herons which are hunting fish and ducks who visit the pond are two of the worst culprits, with both being capable of spreading a wide range of diseases and parasites. The flukes may have been brought by the feathers of the bird, or the eggs may have been present in the faeces (waste) of the animal. Preventing predators and ducks from entering your koi pond will help stop parasites and bacteria from spreading, and will also help with both stress and water quality.

Another common way flukes may get into your water is when you introduce new fish or plants to the pond. It’s almost impossible to tell if a young fish has symptoms of parasites, and even with a skin scape, you may not catch any hiding eggs or larvae.

The same with plants, it’s very difficult to know what exactly is attached to the plant until it enters your pond and starts to spread and cause problems. In many cases, this is just a risk you have to take as a fish keeper, but so long as you understand the signs and symptoms of problems and act accordingly, everything can easily be treated.

Can flukes survive in very cold water?

One of the most common times for koi to get flukes is during torpor in 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 their winter torpor, and provides the safest kick-start to the new year when Spring arrives. Correctly “winterizing” a fish pond is recommended for all pond owners with koi.

Should my koi be quarantined from other fish?

A koi with very advanced flukes can be quarantined for additional treatment if the fish is also suffering from open wounds or ulcers. In these cases, koi would benefit from being placed in a closely monitored holding tank and treated for both bacterial infection and parasites at once. This will reduce stress on the fish and give you more control over water conditions and treatment dosing.

For the early stages of flukes, fish do not need to be quarantined, with it being easier to just treat the pond as a whole to be on the safe side. Even if you perform skin scrapes of all koi and only find flukes on a single fish, this does not mean you don’t have free-swimming larvae or eggs elsewhere in your water.

It is also very easy to miss flukes, as they’re absolutely tiny, so quarantining is often unnecessary and only helpful for treating koi with more advanced symptoms. If your fish are showing signs of flukes, or you’ve identified a single fluke, we recommend performing a full treatment and then adding preventive measures to help protect against flukes in the future.

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Koi Fluke Medicine & Treatment Options

Note:  Always read dosage instructions before treatment, and make sure to turn off any UV clarifiers and remove any carbon filtration as it will neutralize medications. 

Treatments for flukes and most other common parasites include things like praziquantel, malachite green & formalin, potassium permanganate, or superverm. These are all chemical-based treatments with varying degrees of safety, but all can be effective when dosed correctly. Salt is also a natural and effective remedy against flukes, but would require the fish to be individually bathed to work, and would also not be able to treat any free-swimming flukes or eggs in the water.

We personally recommend either a praziquantel based treatment or a malachite green & formalin treatment but would lean more towards the former due to its superior results against flukes in particular. Both are wide-spectrum parasite treatments, being able to combat both kinds of flukes and various other diseases in freshwater fish.

Option 1: Aqua Prazi (Recommended)

Aqua Meds Aqua Prazi is one of the best fluke treatments available, and will quickly work to destroy both gill flukes and skin flukes in fish. It is safe for koi, goldfish, and biological filter media, and should have minimal negative effects on the eco-system when dosed properly.

The medicine contains praziquantel and is primarily designed to kill flukes, but will also work on tapeworms and anchor worms. In terms of dosage, a single dose (10g per 100 gallons) added to the pond will kill all adult flukes within 24 hours of treatment. Afterward, a second follow-up dose is required 3-7 days later to kill any hatching larvae. The full treatment cycle will therefore be 1-2 weeks for total parasite removal. A 25% water change before each dose is recommended for maximum results.

Option 2: EcoLab Disease Treatment (Alternative)

An alternative is Eco Labs Broad Spectrum Disease Treatment, which contains both malachite green and formalin. Even though this medicine is not directed at flukes in particular, we know from experience it can still be very effective. It will be able to kill a wide range of parasites, including flukes, and also treats a range of diseases, such as costia and fungal infections. The treatment is added to the water the same as Aqua Prazi, and can be repeated if necessary.

If your fish is showing signs of injury or has open wounds due to parasites, treatment with a wide-range antibacterial is also recommended to reduce the risk of infection and to allow better healing. We recommend API Pond MelaFix which is great for bacterial infections, damaged gills, ulcers, and open wounds in koi.

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Tips for Preventing Koi Flukes from Infecting Ponds

1) Stop Animals Visiting (Predators/Pests)

Since most parasites are brought into a pond by other animals, one of the best ways to guard against flukes is to prevent animals from entering your water in the first place. Ducks and herons, in particular, can bring all sorts of parasites and diseases to a pond and both can be dangerous to fish.

Herons will actively hunt your fish, causing stress and injury, whereas ducks contribute to sludge, ammonia, and problems with water quality. Both predators and pests can be easily deterred with a bit of preparation and investment, and we would strongly recommend it for koi as they do not react well to changes in environment and water conditions.

• For more information on deterring herons, check here.
• For our guide on stopping ducks from coming to your pond, see our article here.

2) Improve Water Conditions

Flukes and other parasites thrive in low-quality water conditions, and in these same conditions, koi and other fish will become more vulnerable. A pond with bad quality water is more likely to house harmful bacteria, fungi, and parasites in comparison to a well-filtered pond with low ammonia and waste. Keeping water in top quality condition is a main goal in fish keeping, and not only will this prevent parasites from spreading, it also ensures koi are in good health to fight back any problems if they occur.

Maintaining good water quality includes optimizing filtration, cleaning filter media, testing water quality, cleaning out excess waste, and providing plenty of aeration. For further reading, check our dedicated articles below:

• Optimizing and Cleaning Media
• Beneficial Bacteria Benefits
• Testing Water Quality
• Providing Aeration

3) Keep Koi In Optimal Health

Keeping your koi fit and healthy ensures they’re in a good position to fight back any problems that may occur in their environment. One of the best ways you can do that is to ensure you’re feeding your fish top-quality food with the correct nutritional profile. Many “budget” koi foods contain too much of things koi don’t need, and not enough of things they do.

For example, cheaper foods will have far too little protein content, and the sources of protein may be from low-quality sources to save money. Most cheaper foods also contain no vitamins, minerals, or essential oils which koi need to maintain healthy slime coats and strong immune systems.  Picking a top-quality feed is vital to healthy koi, and will help them remain strong throughout their adult life.

• Guide to selecting the right koi food

4) Prepare Pond for Winter

Koi are more susceptible to both harmful bacteria and parasites during torpor/hibernation in winter, as their immune system has slowed along with every other bodily function. Making sure koi are well fed in summer, and correctly “winterizing” your pond will ensure they have the best hibernation period possible and the best kick-start to spring.

• Guide to Winterize Koi Pond