Fish lice (Argulus) are flat, disc-shaped crustaceans, not insects, that clamp onto fish with two suction cups and feed on blood. Adult Argulus foliaceus are about 3 to 7 mm and visible to the naked eye, most often on pond goldfish, koi and wild-collected stock. The life cycle averages 30 to 60 days and eggs in the tank outlast a single dose, so plan repeat treatments and clean the tank or pond too.
Manual removal of visible adults with tweezers. Then treat with diflubenzuron (Dimilin) per the product label. It also kills shrimp, crayfish and other crustaceans, so use a hospital tank if your display holds them. Alternative: Cyromazine (in Fish Lice Treatment products). Repeat treatment in 14 days. UV sterilizer kills free-swimming juvenile lice.
Some Argulus species live in saltwater, but do not reuse a freshwater plan: UF/IFAS says potassium permanganate is not recommended in marine systems. Ask a fish health specialist.
Quarantine pond fish 4-6 weeks. Avoid wild-source feeders. UV sterilizer reduces re-infestation risk.
Estimated cost: $30-60.
Fish lice are crustacean ectoparasites in the genus Argulus, family Argulidae, order Arguloida. They are not insects despite the common name - they are crustaceans, related to crabs, lobsters and shrimp. The two species most aquarists encounter are Argulus japonicus (the most common species in North American and global ornamental supply chains, originally from East Asia but now established globally as an invasive) and Argulus foliaceus (the European native, common in European pond and aquarium stock).
Adult Argulus is unmistakable when seen clearly. The body is flat, disc-shaped, oval, about 3 to 7 mm long and 2 to 4 mm wide in adult and older juvenile A. foliaceus, translucent to greenish-grey, often slightly transparent so the dark gut contents show through. Two large suction cups dominate the underside of the body and are used to attach to the host. A pair of compound eyes sits on top of the carapace. The body has a flexible carapace that allows the parasite to flatten against the host or curl slightly when detached. A small piercing stylet (the mouth-cone) protrudes from the front of the body and is used to penetrate fish skin and inject digestive enzymes.
The parasite moves between hosts and within a host. Unlike anchor worms (Lernaea) which embed permanently, Argulus detaches when full and swims to a new feeding location. A heavily-infested fish may have lice on the body, fins, gill covers, and inside the mouth on the gill arches. The parasite is most often spotted on flanks and the dorsal area where the keeper sees them during routine observation.
Anchor worm (Lernaea cyprinacea). Long thread-like worm 10 to 20 mm long with a Y-shaped tail protruding from a red wound. The anchor head is buried deep in the muscle. Unlike Argulus, anchor worms do not move once embedded and look more like a piece of string stuck to the fish than a disc-shaped parasite.
Gill flukes (Dactylogyrus, Gyrodactylus). Microscopic flatworms not visible without 40x magnification. They cause gill flaring, heavy breathing, and mucus production but never produce visible parasites the way Argulus does. If you cannot see anything but the fish is breathing heavily and flashing, suspect gill flukes.
Fish leeches (Piscicola geometra). Long thin segmented worms 20 to 40 mm. Leeches attach with both ends and move in inchworm fashion. Argulus does not have a segmented body and uses suction cups, not posterior attachment.
Copepod parasites (Ergasilus). Tiny white "worms" 1 to 2 mm visible on the gills. Smaller than Argulus and gill-specific.
Camallanus worms. Internal nematode parasites with red worms protruding from the fish vent. Internal parasite, not external like Argulus.
Argulus has a typical aquatic crustacean life cycle: egg, free-swimming metanaupliar larva, on-host juvenile through a long series of molts, on-host adult. UF/IFAS puts the average cycle at 30 to 60 days, depending on species and temperature, with faster cycling in warmer water. This life cycle is critically important for treatment - most treatments only kill one life stage, so you must time doses to catch each generation as it appears.
| Stage | Age or timing | Where it lives |
|---|---|---|
| Egg | 10 days at 35 C to 61 days at 15 C (A. japonicus); 17 days at 23 C (A. foliaceus) | Hard surfaces and plants |
| Metanauplius larva | Must find a host within 2 to 3 days in many species | Free in the water |
| Juvenile | 11 molts in A. foliaceus; sexual maturity about 30 to 40 days after hatching | On the fish |
| Adult | Keeps molting; survives several days off the fish | On the fish; females leave to lay eggs |
Source: UF/IFAS publication FA184, Argulus (Fish Louse) Infections in Fish.
Sexually mature female Argulus leaves the fish host to deposit eggs on hard surfaces - rocks, plants, pond walls, filter media, sometimes the tank glass. Eggs are laid in clusters or strings of 20 to 250, glued in place with a cement-like secretion that resists scraping. After laying, the female returns to a fish.
Hatch time depends on species and temperature: A. japonicus eggs take 10 days at 35 C and 61 days at 15 C, while A. foliaceus eggs take 17 days at 23 C and 30 days at 20 C. Eggs laid in fall can overwinter and hatch the next spring, which is why pond outbreaks are seasonal.
Hatched larvae are tiny (under 1 mm) free-swimming metanauplii that, in many species, must find a fish host within 2 to 3 days or die. Larvae use chemical cues (fish kairomones) and physical cues (water disturbance) to locate hosts. This is the only life stage during which Argulus is fully free-swimming and vulnerable to UV sterilizers and substrate cleaning.
Larva attaches to the host, begins feeding, and molts through a series of juvenile stages (11 molts in A. foliaceus), reaching sexual maturity roughly 30 to 40 days after hatching. Unlike copepod parasites, Argulus keeps molting after maturity. Each molt requires the parasite to shed its old carapace and grow a new one - this is the vulnerability window for chitin-synthesis-blocking drugs like diflubenzuron. Juveniles are smaller than adults (1 to 4 mm) and harder to spot during inspection.
Sexually mature adults feed, mate (on host or in water column near a host), and females then depart to lay eggs. Adults can survive off-host for several days, making them resistant to environmental drying or fish transport intervals shorter than their off-host tolerance.
Diflubenzuron (Dimilin) interferes with molting. UF/IFAS reports it kills both adult and larval stages, but eggs on tank or pond surfaces are the gap in any single dose. Eggs can take 10 to 61 days to hatch depending on temperature, so one treatment looks like it worked and then new lice appear. UF/IFAS says treatment should target eggs, juveniles and adults, on the fish and in the environment. That means repeat treatments on the label schedule plus cleaning, disinfecting or drying the original system.
Diflubenzuron is the active ingredient in Dimilin and several branded pond products (Anchors Away, Microbe-Lift Lice and Anchor Worm). It is a benzoylurea insecticide that blocks chitin synthesis during arthropod molting. Argulus keeps molting through its whole life, so exposure disrupts the exoskeleton and UF/IFAS reports it kills both adult and larval stages.
Dosing. Follow the product label and calculate from your real water volume. Diflubenzuron is a restricted-use pesticide, applicators must follow the label, and there are no FDA-approved drugs for Argulus, so check the rules in your state and talk to a fish health specialist first.
Re-dosing. Plan repeat treatments on the label schedule to catch lice hatching from eggs, which takes longer in cool water.
Compatibility. Safe with fish and biofilter bacteria. NOT safe with shrimp, crayfish, snails, or any other invertebrate - diflubenzuron is broadly arthropod-toxic. Use a separate hospital tank if your display contains inverts you want to preserve.
Cost. 30 to 80 dollars per pond-size treatment. Aquarium-scale doses are cheaper.
Emamectin benzoate (SLICE) is an in-feed drug licensed outside the US for sea lice, and it has controlled Argulus experimentally in goldfish and koi.
Availability. In the US it is an FDA Investigational New Animal Drug, so it is unavailable for general use and requires enrollment in an INAD study through the manufacturer.
Use case. Not a realistic option for typical aquarium or pond keepers in the US.
Cyromazine works on the same mechanism as diflubenzuron but with slightly different efficacy profile. Some pond products combine both for redundancy. Dose and treatment schedule similar to diflubenzuron.
For tanks with under 5 visible adults, manual removal with fine-tip tweezers under fish sedation is faster than waiting for drug treatment. Sedate the fish with clove oil (1 drop per liter in a small bath) for 2 to 4 minutes until the fish becomes calm and unresponsive. Lift the fish from water with a wet towel, locate each Argulus, grip with fine tweezers, pull straight up (don't twist - the stylet can break off in the wound). Dab the wound with povidone-iodine (Betadine) and return the fish to a recovery container.
Manual removal is not effective for large infestations because the parasite moves between fish, and you cannot catch eggs deposited on tank surfaces. Always combine manual removal with diflubenzuron for full eradication.
KMnO4 at 10 mg/L for 30 minutes in a separate dip container kills adult Argulus on contact through oxidative damage. Use only on hardy pond fish (koi, goldfish, large minnows). Never use on scaleless catfish, loaches, sensitive tetras, or any tropical aquarium fish.
KMnO4 stains skin, fabric, and plastic purple permanently - use in a dedicated dip container, wear gloves, and dispose of the spent solution in a contained drain. Adding hydrogen peroxide (3 percent solution) at 1 mL per 10 L of spent solution neutralizes the purple color.
KMnO4 does not affect eggs. Combine with diflubenzuron for full eradication.
An organophosphate that kills Argulus directly. UF/IFAS reports Dylox 80, the only aquatic organophosphate approved for this use in Florida and several other states, is no longer made. Trichlorfon is toxic to plants, snails, shrimp, and stressed fish. Use only as a pond treatment under veterinary guidance.
UF/IFAS reports that increased salt concentration does not appear to control Argulus in freshwater, and neither does formalin. Do not rely on either.
A dedicated hospital tank for Argulus treatment is recommended whenever your display contains invertebrates (shrimp, snails, crayfish) that would be killed by diflubenzuron, or when treating a single high-value fish requires more concentrated dosing and observation than the display allows. The hospital tank does not need to be elaborate - a 10 to 40 gallon bare-bottom tank with cycled sponge filter and heater is sufficient.
While the fish patient is in the hospital tank, the display tank must also be treated to break the parasite life cycle. Egg clusters on glass, rocks, and decor will hatch new lice over the next 30 to 60 days. Options:
Prevention is dramatically cheaper than treatment. Once Argulus is established in a tank or pond, the 30 to 60 day treatment cycle, hospital tank costs, and risk of spring viremia of carp make prevention the only sensible long-term strategy.
Ponds are the natural Argulus habitat in temperate climates. Wild waterfowl can introduce eggs on feet or feathers and any open pond connected to other water sources (creek, rainwater runoff from a neighbor's contaminated pond) can introduce parasites. Pond treatment requires correct gallon estimation - dosing errors in either direction are a risk with a restricted-use pesticide, and under-dosing wastes a treatment cycle.
Pond infestations often cycle seasonally with temperature. Spring egg-hatch produces a parasite explosion that peaks in midsummer and declines as temperatures cool. UF/IFAS notes infestations tend to peak in summer and fall, so inspect fish closely as water warms and act on the first lice you see.
Argulus is uncommon in indoor aquariums because the supply chain is shorter and aquarium fish are typically inspected more carefully. When Argulus does appear in an aquarium, the source is almost always: (1) wild-collected feeder fish (rosy reds, comet goldfish); (2) plants transferred from a pond; (3) fish purchased from a chain store that mixes pond and aquarium stock; or (4) substrate or decor moved from a pond.
Aquarium treatment is easier than pond treatment because the smaller volume is cheaper to dose and decontaminate. The hospital tank setup described above works for any aquarium-scale infestation.
Adult Argulus can survive for several days off the fish. In many species, larvae must find a host within 2 to 3 days of hatching or they die. Eggs last far longer: they can take 61 days to hatch at 15 C and can overwinter in ponds. A fishless period must outlast the hatch time at your water temperature, so 30 days is not enough in cool water.
Fast enough that a light infestation can escalate quickly. Argulus eggs hatch in as little as 10 days at 35 C or 17 days at 23 C, and juveniles reach sexual maturity roughly 30 to 40 days after hatching, according to UF/IFAS. Females leave the fish to lay eggs on hard surfaces and plants, then return to a host. Treat as soon as you identify lice rather than waiting to see if it gets worse.
Three possibilities. First, residual treatment irritation - fish often flash for 1 to 2 weeks after treatment due to skin recovery. Second, secondary bacterial infection at the puncture sites - treat with Furan-2 or Kanaplex. Third, a parallel parasite (gill flukes, ich) masked by the Argulus diagnosis - inspect gills under sedation and treat if found.
Yes. Warmer water (26 to 28 C) accelerates the parasite life cycle and shortens the treatment timeline because each generation hatches and molts faster. Cool water (under 20 C) slows everything down and extends the treatment to 45 to 60 days. Treating at the fish's preferred temperature is recommended unless your veterinarian advises otherwise.
No. UF/IFAS states there are currently no FDA-approved drugs for treating and controlling Argulus. Diflubenzuron is a restricted-use pesticide, so applicators must follow the label, and legality varies, especially for food fish. Of the options UF/IFAS lists, only potassium permanganate in freshwater is legal for fish intended for human consumption. Work with a fish health specialist, and check your state rules before you treat a pond.
No. Diflubenzuron disrupts molting, which is how it kills fish lice, so it also harms shrimp, crayfish and other crustaceans that molt. Do not dose a tank that holds them. Move the affected fish to a bare hospital tank, remove carbon from the filter for the treatment, and clean the display separately. Scrubbing surfaces and vacuuming substrate is the fallback for displays where the invertebrates have to stay put.
No. Argulus is fish-specific and cannot survive on warm-blooded animals, so removing lice by hand with tweezers poses no human health risk. Despite the common name, fish lice are crustaceans, related to crabs and shrimp, and are nothing like the lice that affect people. Wash your hands after working in the tank, as you would after any aquarium maintenance.
The single most important disease-prevention step: a 4-6 week quarantine of every new fish before adding to your display. See the complete quarantine protocol.