Koi Fish Lice (Argulus): How to Spot Them and What to Do
General guidance — not a diagnosis. When in doubt, call your vet.
A moving spot deserves attention
A small, flattened parasite moving over your koi's skin may be a fish louse, or Argulus. Fish lice are branchiuran crustaceans related to crabs, lobsters, and shrimp—not true lice or worms.[1] They are especially associated with wild and pond-raised freshwater fish, including koi, goldfish, and other cyprinids.[1]
Seeing an oval speck is suspicious, but it is not enough to choose a treatment. Older Argulus stages can be visible without magnification, yet UF/IFAS recommends that identification be verified by a fish health professional.[1] Photograph the spot, observe whether it moves, and arrange confirmation before adding medication to the pond.
What fish lice look like
Fish lice have a flattened, oval-to-rounded body. Adults attach with suckers and pierce the skin with a stylet to feed; they can detach, move over a fish, or swim through the water.[1] Merck likewise describes branchiurans as flattened parasites that move rapidly over the skin and periodically attach using hooks and suckers.[2]
Inspect the skin, fins, mouth, and gill chamber without repeatedly chasing or netting the fish. Visible parasites may be scattered or clustered, including around the head. The attachment site may show a pinpoint hemorrhage, excess mucus, fin or scale loss, or local inflammation. Affected fish may flash against surfaces, swim erratically, eat less, hang near the surface, or lose condition.[1]
A louse looks different from the threadlike female copepod described in aquarium fish anchor worm. Microscopic flatworms can also cause irritation and flashing; see goldfish flukes. If you see an open lesion but no identifiable parasite, use the broader triage guide for goldfish ulcers and red sores.
Check every fish and the pond
Observe all fish for appetite, breathing, balance, rubbing, mucus, sores, and visible parasites. Record when the problem began and whether new fish, plants, water, nets, or other wet equipment recently entered the system. Test and record temperature, pH, ammonia, nitrite, nitrate, and dissolved oxygen if you have suitable equipment. Poor water quality does not prove fish lice, but unstable conditions can make an affected fish less able to cope.
Older Argulus may be seen moving on the host or swimming in the water. A veterinarian or fish health professional can examine the fish promptly and use a wet mount of affected tissue to support identification.[1] This matters because irritation, hemorrhage, excess mucus, and abnormal swimming are not unique to fish lice. Treating only by appearance can miss another parasite, a water-quality problem, or a secondary infection.
If fish can be moved without destabilizing temperature or water chemistry, keep newly acquired or affected fish separate in a mature quarantine system. Use dedicated nets, tubs, siphons, and towels. Do not transfer pond water, wet plants, or equipment into an unaffected system. Maintain steady aeration and filtration while arranging help.
Why removal is not enough
An adult louse can leave the fish and lay eggs on hard surfaces or vegetation.[1] That is why plucking off one visible parasite cannot clear the pond. UF/IFAS calls manual removal impractical in many situations and incomplete because eggs and unattached juveniles or adults can remain in the environment.[1]
Do not apply pesticides, prescription antiparasitics, concentrated salt, peroxide, or antiseptics directly to the fish or pond without case-specific guidance. Product choice depends on fish species, system volume, water chemistry, plants, invertebrates, filtration, local rules, and whether fish are intended for food. UF/IFAS states that there are no FDA-approved drugs for treatment and control of Argulus and recommends working with a fish health specialist.[1]
A clinician may confirm the parasite, assess the gills and damaged skin, look for concurrent disease, and build a plan that addresses fish and environmental stages. Keep the system stable and document new lesions or deaths rather than repeatedly changing products.
When to See a Vet
- Seek urgent help for gasping, very rapid breathing, loss of balance, collapse, pale gills, heavy bleeding, or repeated deaths.
- Arrange prompt assessment when lice or wounds involve the eye, mouth, or gill chamber, or when a sore is spreading or producing heavy mucus.
- Contact a fish veterinarian when multiple fish are flashing, not eating, or developing visible parasites.
- Get professional advice before using any pesticide or antiparasitic, especially in ponds with invertebrates or fish intended for food.
For fish owners
An article can't ask about your fish.
Describe what you're seeing. You'll get a few follow-up questions about your fish, then where it lands — vet today, watch closely, or no visit needed — and what to do at home meanwhile.
Frequently Asked Questions
Are fish lice actually lice?
No. UF/IFAS describes Argulus as a branchiuran crustacean related to crabs, lobsters, and shrimp.[1]
Can I see fish lice without a microscope?
Older stages may be visible moving on a fish or swimming in the water. UF/IFAS still recommends having the identification verified by a fish health professional; a wet mount of affected tissue can help.[1]
Can I pull a fish louse off my koi?
Removing one visible parasite is not a complete solution. UF/IFAS explains that eggs and unattached juveniles or adults may remain in the environment, so management must address more than the parasite you can see.[1]
Why is my koi flashing after I removed the louse?
Flashing means the fish is irritated, but it does not identify the cause. More fish lice may remain, the attachment site may be injured, or another skin, gill, or water-quality problem may be present.
Can fish lice cause sores?
Yes. UF/IFAS describes localized inflammation and damage at attachment sites and notes that this damage may lead to secondary infections.[1]
Should I treat the whole pond?
Treat this as a pond-level problem, but do not choose or dose a product by guesswork. Fish lice have stages on the fish and in the environment, and a fish health professional should tailor control to the species and system.[1]
References
- Steckler and Yanong, UF/IFAS Extension, 2025 — Argulus (Fish Louse) Infections in Fish. https://ask.ifas.ufl.edu/publication/FA184
- Petty, Francis-Floyd, and Yanong, Merck Veterinary Manual Professional, 2022 — Parasitic Diseases of Fish. https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/parasitic-diseases-of-fish