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.