How to Treat Fish Lice (Argulus) in Aquariums
Spotting a flat, disc-shaped parasite clinging to your fish is alarming — and rightly so. Fish lice, scientifically known as Argulus, are crustacean ectoparasites that attach to skin and fins, feeding on blood and tissue fluid. Knowing how to treat fish lice argulus aquarium infestations early prevents serious damage to your livestock. At Gensou Aquascaping in Singapore, we have encountered Argulus on everything from goldfish to wild-caught cichlids, and over 20 years of hands-on experience has refined our treatment approach.
Recognising Argulus on Your Fish
Argulus are visible to the naked eye — adults measure 5-10 mm across and resemble tiny, translucent green or brown discs. They move across the fish’s body, often settling near the base of fins or along the flanks. Infected fish flash against rocks and substrate, clamp their fins, and may develop red, irritated patches where the parasite has been feeding. Look carefully during water changes when fish are calmer and closer to the glass.
Life Cycle and Spread
Female Argulus detach periodically to lay gelatinous egg strings on hard surfaces — rocks, wood, even filter intakes. Eggs can hatch in as little as two weeks at Singapore’s typical water temperatures of 27-30 °C, releasing free-swimming larvae that seek a host immediately. This rapid cycle means a single overlooked parasite can produce hundreds of offspring within a month. Pond fish, new arrivals, and wild-caught specimens are the most common sources of introduction.
Manual Removal
For light infestations, manual removal is the fastest first step. Net the affected fish gently and place it on a damp towel or in a shallow container with tank water. Using fine-tipped tweezers, grasp the louse at its edge and pull firmly but steadily. The parasite’s suckers will release. Dab the attachment site with a cotton bud moistened with diluted povidone-iodine to prevent secondary bacterial infection, then return the fish to the tank promptly. Work quickly — the fish should be out of water for no more than 30 seconds.
Chemical Treatments
When multiple fish are affected, chemical treatment is necessary. Dimilin (diflubenzuron) is the most targeted option — it inhibits chitin synthesis, killing larvae and preventing eggs from developing, while being safe for fish at recommended doses of roughly 0.01 mg per litre. Potassium permanganate baths at 10 mg per litre for 30 minutes can dislodge adult parasites but must be monitored carefully to avoid gill damage. Organophosphate-based treatments are effective but highly toxic to invertebrates, so remove shrimp and snails before dosing.
Cleaning the Tank Environment
Removing adult lice from fish is only half the battle. Scrub hard surfaces where egg strings may be deposited — rocks, driftwood edges, and filter housings. Vacuum the substrate thoroughly during each water change. If the infestation is severe, consider removing and scrubbing hardscape in hot water above 60 °C to destroy eggs. Replace filter sponges or rinse them in dechlorinated water at elevated temperature. This environmental cleanup breaks the breeding cycle more effectively than medication alone.
Quarantine and Prevention
Every new fish should spend a minimum of three weeks in quarantine before entering your display tank. Inspect new arrivals closely under good lighting — Argulus are large enough to see without magnification. Avoid adding water from transport bags, as free-swimming larvae may be present. Plants from outdoor ponds can also carry eggs; give them a brief alum dip (one tablespoon per litre for two hours) before planting. These precautions cost almost nothing but save considerable trouble later.
Monitoring Recovery
After treatment, watch for secondary infections at bite sites. Argulus wounds are entry points for bacteria like Aeromonas and Pseudomonas, which can cause ulcers if left untreated. A mild antibacterial treatment or salt bath (3 g per litre for 15 minutes) supports healing. Continue checking fish daily for at least four weeks — any missed eggs will hatch and reattach. If you need hands-on support diagnosing or managing an Argulus outbreak in Singapore, Gensou Aquascaping is happy to assist with on-site tank assessments.
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