Fish lice (Argulus) are flat, disc-shaped crustaceans that attach to fish with suction cups + feed on blood. Adults reach 5mm and are visible to the naked eye. Common in pond fish (goldfish, koi) and wild-collected stock.
Manual removal of visible adults with tweezers. Then dose entire tank with Dimilin (diflubenzuron) - kills lice but is invertebrate-safe in low doses. Alternative: Cyromazine (in Fish Lice Treatment products). Repeat treatment in 14 days. UV sterilizer kills free-swimming juvenile lice.
Saltwater lice (different species) treated similarly.
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 aquatic crustaceans more closely related to copepods than to terrestrial lice. 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, 5 to 10 mm long for adults, 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 5 to 9 molts, on-host adult. The whole cycle from egg-laying to a new sexually-mature adult takes 40 to 90 days depending on temperature, with faster cycling at warmer temperatures. 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.
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. A single female lays multiple egg batches over her 2 to 6-month lifespan, totaling 500 to 2,000 eggs.
Eggs hatch in 10 to 30 days at 20 to 25 C, faster at higher temperatures (5 to 10 days at 28+ C), much slower at cool temperatures (up to 60 days at 12 C). Eggs in ponds typically overwinter and hatch in spring when water warms, which is why pond Argulus outbreaks are seasonal and predictable.
Hatched larvae are tiny (under 1 mm) free-swimming metanauplii that must find a fish host within 24 to 48 hours or starve. 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 5 to 9 successive juvenile stages over 30 to 60 days. 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) only kills molting juveniles - it has no effect on adults or eggs. Cyromazine works similarly. This is why every effective treatment plan involves multiple doses spaced 14 to 21 days apart: dose 1 kills the juveniles currently molting, dose 2 kills the next generation from eggs that hatched after dose 1, and dose 3 (if needed) kills any final stragglers. A single dose appears to work for 7 to 14 days (visible adults die from old age, eggs are not yet hatching), then the next generation explodes.
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 juveniles attempting to molt under diflubenzuron exposure fail to form a new exoskeleton and die. Adults are not directly killed but cannot reproduce successfully if their offspring all die in the molt cycle.
Dosing. 0.066 mg/L of active ingredient. Product labels vary in concentration - calculate based on the active ingredient percentage. Most pond products formulate to dose 1 oz per 500 gallons or similar.
Re-dosing. Re-dose every 14 to 21 days for 2 to 3 cycles to catch each new generation hatching from eggs.
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 (sold as Slice for veterinary use and in some compounded medicated feeds) is the standard treatment in commercial salmonid farming for sea lice and the most effective single treatment for Argulus. It is fed in medicated food at 50 micrograms per kg fish body weight daily for 7 days. The drug accumulates in fish blood and reaches Argulus through normal feeding.
Availability. Veterinary prescription required in most US states. Difficult to source for hobbyist use. Aquatic veterinarians (find one through the World Aquatic Veterinary Medical Association directory) can prescribe and compound it.
Use case. Best for valuable koi or pond stock where veterinary involvement is justified by fish value. Not practical for typical aquarium use.
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 older organophosphate that kills Argulus directly. Highly effective but increasingly restricted in many regions due to organophosphate concerns. Trichlorfon is toxic to plants, snails, shrimp, and stressed fish. Use only as a pond treatment under veterinary guidance.
Aquarium salt at 0.3 percent (3 g per liter) over several weeks may slow Argulus reproduction by reducing larval survival. Not effective as a sole treatment but useful as a supportive measure during diflubenzuron treatment.
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 viral co-infection (SVCV, KHV) 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, mosquito larvae are not vectors but their predator fish often are, 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 - over-dosing diflubenzuron in a small pond is safe but expensive; 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. Treating in early spring (when water reaches 60 F) catches eggs before they hatch and prevents the summer peak.
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 3 to 7 days off-host, depending on temperature. Larvae must find a host within 24 to 48 hours of hatching or starve. Eggs survive indefinitely on hard surfaces until temperature triggers hatching. This is why fish-out periods in the display tank during treatment need to be at least 30 days - any free-swimming larvae will die within 2 days, but you need to wait for all eggs to hatch.
A single mated female lays 500 to 2,000 eggs over her lifetime. At 25 C, eggs hatch in 10 to 14 days. Juveniles reach reproductive maturity in 30 to 40 days. The math: 10 adult Argulus introduced to a tank can become 2,000+ within 90 days if untreated. This is why fast intervention matters - waiting "to see if it gets worse" guarantees that it will.
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. Argulus is fish-specific and cannot survive on warm-blooded animals. Manually removing them poses no human health risk.
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.