Flukes in Koi Fish – Causes, Symptoms & Treatments

There are all kinds of things that can go wrong when you keep fish, especially pond fish. They are exposed to the elements and predators, and they are at a higher risk of parasites and illnesses. This is partially due to the difficulty of maintaining high water quality in a pond and partially due to the risk of exposure to parasites and pathogens that exist in nature. If you’re concerned that your koi fish might have flukes, then keep reading for everything you need to know about these parasites.

What are Flukes?

There are two types of flukes that are commonly seen infreshwater fish like koi. The first is monogeneans from the Dactylogyrus genus. This parasite is most commonly found on the gills of affected fishes. The second type of flukes belong to the Gyrodactylus genus and are most commonly found on the skin. Monogeneans are a large group of parasites containing thousands of species. There are around 1,500 species that may affect different types of fish.

Both types of flukes are microscopic parasites that attach themselves to the uppermost layers of tissue and begin feeding from the fish. Gill flukes can measure up to 3mm (0.12 inches) in length, but they are clear and still too small to be easily seen with the naked eye. Skin flukes only grow to around 0.4mm (0.016 inches) in length, so they are far too small to be seen with the naked eye. Dactylogyrus species are more common in koi and other cyprinids than Gyrodactylus are. Both types of flukes can be deadly through the risk of secondary infections and, in the case of gill flukes, excessive mucus production in and around the gills can lead to suffocation.

What Causes Flukes?

It takes more than the presence of flukes for them to take hold of your fish. Healthy fish typically have an immune system that helps to protect them against parasitic infections, like flukes. However, overstocking, poor filtration, poor water quality, illness, and other environmental stressors can all increase the risk of your fish becoming infested with gill or skin flukes. These stressors lead to immune system depression, which always increases the risk of infections and secondary infections in your fish. Flukes may be a primary illness in your fish, but they can also opportunistically infect a fish that has been weakened by another illness.

If you were thinking that maybe your koi would be safe from flukes during the winter, you’ll be disappointed to know that winter is a common time for koi to become infected with flukes. The fluke life cycle slows down significantly in cold water, but it continues to occur in water as cold as just above freezing. When the water is cold, your koi are in torpor, which is a semi-hibernation state during which their metabolism drops significantly. This leads to a drop in the immune system as well, which makes the more susceptible to fluke infestations than they are when they are healthier and more active in warm water.

What are the Symptoms of Flukes?

The good news is that identifying fluke infestations is typically relatively easy, although they cannot be truly diagnosed without a skin scrape viewed under a microscope. Lethargy, inappetence, and flashing are all common indications of a parasitic infection. Flashing consists of the fish swimming rapidly and irregularly around the tank, often rubbing up against items as it does so.

When you look at your fish, you won’t see the flukes, but you will see the damage the flukes are doing. Scale loss, areas of redness, excessive slime coat production, bleeding, bruising, and ulcerations in the skin are all easily noticeable symptoms of flukes. With gill flukes, you may also note your fish making a chewing motion or seeming to be breathing more quickly or labored. Less noticeable symptoms may include hiding, color loss, and redness or ulcerations on the eyes if the flukes have infected this area.

How Do You Treat Flukes?

There are a variety of ways you can treat flukes, but there are a handful of treatment options that rise above the rest due to their high efficacy. Here are some of the best products you can use to treat flukes in your koi.

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	Image	Product	Details
Best Overall		Hikari Prazipro	16-ounce bottle treats up to 1,920 gallons of water Can be used to treat the pond or as a medicated bath Non-toxic and safe for most plants and aquatic animals	CHECK PRICE
		Eco Labs Pond Microbe-Lift	Highly effective Can be used in cool water Both components work synergistically and reduce the toxicity risk of each other	CHECK PRICE
		Crystal Clear ParaSalt Pond Salt	100% sodium chloride dehydrates the parasites Increases electrolyte concentration in the water Aids in wound healing and supports slime coat production	CHECK PRICE

1. Hikari Prazipro

Praziquantel based products are the best option for treating flukes, and Hikari is a well trusted brand in the aquatics world. A single 16-ounce bottle treats up to 1,920 gallons of water and this product is non-toxic and safe for most plants and animals, including koi and other pond life. It’s important to use this product for the full recommended length of treatment, though, or it may not be effective. It can be used as a whole pond treatment or a medicated bath.

2. Eco Labs Pond Microbe-Lift

A highly effective combination of chemical treatments against flukes is malachite green and formalin, which are the active ingredients in the Eco Labs Pond Microbe-Lift. Both of these chemicals can have high levels of toxicity if not administered properly but combining them and reducing the individual dosages of each component significantly reduces the toxicity risks. However, proper dosing is still extremely important to the safety and survival of your fish. This product can be used in water as cool as 50°F, making it the only type of medication that is safe to use in late fall and early spring. It cannot be safely combined with other medications or salt. A 32-ounce bottle treats up to 9,600 gallons of water.

3. Crystal Clear ParaSalt Pond Salt

The Crystal Clear ParaSalt Pond Salt is effective against most parasites and can be used as a preventive or a treatment, but it is less effective than stronger chemical treatments. It consists of 100% sodium chloride and works by dehydrating the parasites. If dosed inappropriately, it can be damaging to your fish and salt is often not recommended for use with plants and many types of invertebrates. A 10-pound container of ParaSalt can treat up to 2,000 gallons of water. It increases the electrolytes in the water and aids in wound healing, keeping your fish healthier.

In Conclusion

Dealing with flukes in your koi pond can be a big mess, and you may lose some fish if the flukes have had time to take hold. Flukes can be deadly to your fish, so it’s important to closely monitor your pond fish to catch wounds and unusual behavior early. Treating flukes is doable, but it does take the proper tools and closely following instructions on any products you use. Inappropriate dosing can damage your fish, plants, and pond biome, so make sure you’re using products correctly to treat flukes.

Featured Image Credit: Jerawat Supajirakit, Shutterstock