Fish lice (Argulus): how to spot them, remove them and treat your tank or pond

Fish lice (Argulus) are flat, round crustaceans, a few millimetres across, that cling to fish and feed by piercing the skin. Pick off what you can see, then treat the whole tank or pond for several weeks, because their eggs are stuck to glass, rocks and plants and keep hatching.

They're most common on goldfish, koi and other pond fish in summer and autumn. Salt baths, a common home remedy, don't reliably kill them.

How to identify it

A flat, round or oval disc, about 3 to 7 mm (1/8 to 1/4 inch), stuck to the skin or fins. Often clear or greenish-grey, so it's easy to miss.
Two dark eye spots and two round suckers on the underside, visible with a magnifying glass.
It moves. Lice can slide across the fish and swim off to another fish, especially when the fish is netted.
Signs on the fish: tiny red pinpoint spots, red patches, extra slime, frayed fins, missing scales, rubbing against objects (flashing), jumping, erratic swimming, hanging at the surface and poor appetite.

Easily confused with: a scale, a small air bubble, or a leech. Leeches are longer and stretch and contract like a worm; lice stay flat and round. Anchor worms look like threads, not discs.

Tip: lice often let go when a fish is lifted from the water, so check fish in a clear container of tank water, and look in the net afterwards.

When it's a problem

Normal: fish lice are never normal in an aquarium or pond.
Early: one or two lice spotted on one fish.
Serious: lice on several fish, red patches, fish rubbing and jumping.
Emergency: small fish with several lice, open sores, fish gasping or lying still, or sores turning white and fuzzy. Heavy infestations can kill small fish, and the wounds let in bacteria and fungus.

What to do right now

Check every fish in a clear container or with a torch (flashlight).
Remove visible lice with fine tweezers, holding the fish gently in a wet net. Work quickly; seconds out of water, not minutes.
Test the water and do a water change if ammonia or nitrite is above zero.
Move badly wounded fish to a clean hospital tank.
Look for eggs: small rows or patches of eggs on glass, rocks, wood, plant stems and pond liners.
Get a treatment that targets crustacean parasites and plan to treat the whole system.

What to do next

Understand the life cycle. After mating, females leave the fish to lay eggs on hard surfaces and plants, then return to a fish. Eggs take about 17 days to hatch at 23°C (73°F) and about 30 days at 20°C (68°F), and eggs laid in autumn can last until spring. Newly hatched lice must find a fish within two to three days or die. Adults can survive off a fish for several days. The full cycle takes about 30 to 60 days, so plan for a long treatment.

Treatment options by active ingredient. The University of Florida notes there are no FDA-approved drugs for fish lice in the US, and rules vary between countries. Use a product sold for fish lice or anchor worms, follow the label, and ask a vet if unsure.

Diflubenzuron and lufenuron ("chitin inhibitors"): stop lice from building their outer skeleton, so they die when they moult. They work on young and adult stages. They kill shrimp and other crustaceans, and diflubenzuron may be restricted-use where you live.
Organophosphates (such as trichlorfon): used in the past, often given weekly for several weeks. Some products are no longer made, use is restricted, and they're toxic to people as well as fish.
Potassium permanganate: can work in freshwater, but it's easy to overdose and is hard on gills. Use only with vet guidance.
Emamectin (in food): shown in experiments to work in goldfish and koi, but generally only available through a vet, if at all.
Not effective: raising salt levels and formalin, according to University of Florida experts.

Deal with the eggs. In an aquarium, scrub the glass, rocks and decor during water changes, and remove egg patches with a scraper. For a severe outbreak in a small tank, move the fish to a hospital tank, then empty, clean and fully dry the original tank and decor before setting it up again.

General cautions. Remove activated carbon while medicating. Keep shrimp, crayfish and snails out of treated water. Be careful with scaleless fish, plecos and other catfish, and loaches. Watch ammonia and nitrite, as treatments can affect the filter bacteria. Finish the full course, even if the lice seem gone.

Recovery. Watch the bite marks for 2 to 3 weeks. Sores that spread, turn white or fuzzy suggest bacterial infection (see fin rot and columnaris). Antibiotics should be chosen with a vet; many countries require a vet prescription.

Ponds. Lice are most common in ponds and in fish bought from ponds. Calculate your pond's volume carefully, treat all connected water, and remember that treatments for lice also harm insects, frogs' food and other wildlife. Fish lice can also carry diseases between fish, including spring viraemia of carp, a serious reportable disease of koi and goldfish. Get expert help for large or valuable ponds.

Products that help

Fine-tipped tweezers: for picking lice off fish.
Treatment for crustacean parasites: sold for fish lice and anchor worms. Usually based on a chitin inhibitor.
Clear container and magnifying glass: make lice and their suckers easier to see.
Algae scraper and stiff brush: remove egg patches from glass and decor.
Hospital tank: keeps wounded fish safe while you treat.
Water test kit: keep ammonia and nitrite at zero during treatment.
Keep on hand: a quarantine tank, dechlorinator and a fine net.

How to prevent it

Quarantine new fish, especially goldfish, koi and pond fish, for at least four weeks, and check them closely.
Look for lice on fish before buying.
Don't add pond water, wild plants or wild-caught live food to aquariums.
Don't add shop water to your tank or pond.
Keep pond and aquarium nets and equipment separate, or dry them fully between uses.

Why it happened

Fish lice nearly always arrive on a new fish, in pond water or plants, or with wild-caught live food. Once inside, females keep laying eggs on surfaces, so a few lice can turn into a lasting problem.

When to get help

Get help from a fish vet if many fish are affected, if you have a pond or valuable koi, if wounds become infected, or if fish keep dying. A vet can confirm the parasite with a microscope and tell you which treatments are legal and safe where you live.

In the game

Multiple Tank Syndrome doesn't have a fish lice card yet. The "Quarantine tank" card is the best defence against parasites riding in on new fish.

Sources

Steckler N, Yanong RPE. Argulus (fish louse) infections in fish (FA184). UF/IFAS Extension.
Merck Veterinary Manual. Parasitic diseases of fish.
Merck Veterinary Manual. Management of aquarium fish (quarantine).