What does Argulus / Fish louse (Argulus spp.) look like?

Visible 5-7mm flat parasites on body. Scratching/flashing. Red lesions where lice attach.

How is Argulus / Fish louse (Argulus spp.) treated?

Dimilin / Lufenuron: Insect growth regulator. Single dose treats tank-wide. Disrupts molting; effective + low-toxicity to fish.

POND disease

Argulus / Fish louse (Argulus spp.)

Reviewed by the Fast Aquatics husbandry team · Updated May 2026

Quick referenceArgulus (fish lice) are flat, disc-shaped crustaceans that attach to fish skin and feed on blood. Common in pond + outdoor ponds; treat with Dimilin/Lufenuron or manual removal.

Symptoms

Visible 5-7mm flat parasites on body
Scratching/flashing
Red lesions where lice attach
Color fade
Lethargy
Refusing food

Cause

Argulus crustaceans (often A. foliaceus, A. coregoni). Free-swimming larvae attach to fish + suck blood. Introduced via wild-caught feeders, plants, or pond water.

Treatment options

Dimilin / Lufenuron. Insect growth regulator. Single dose treats tank-wide. Disrupts molting; effective + low-toxicity to fish.
Manual removal. For visible adults: tweezers + iodine on attachment site. Catches what you see; doesn't solve infestation.
Salt bath. 3% saltwater dip for 5-10 minutes (dechlorinated water + non-iodized salt). Crustaceans drop off.
Parasite-S (trichlorfon, organophosphate). Effective but toxic to scaleless fish + shrimp. Use only in dedicated pond systems.

Always treat in a separate quarantine or hospital tank. Most medications are toxic to coral, invertebrates, and live rock biology. Consult an aquatic veterinarian for valuable fish.

Prevention

Quarantine new fish. Disinfect new plants in alum or potassium permanganate dip. Don't mix wild-caught feeder fish with show koi.

Fatality + outcome

Low to moderate - blood loss + secondary infections kill weak/small fish. Healthy adult koi survive but lose color + condition.

Full disease database · Symptom matcher · Q&A library · Glossary · QT timeline calculator

Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.