Fish Lice (Argulus)

This guide covers fish lice Argulus aquarium with practical steps for home aquariums. Fish lice look like tiny flat ticks on skin — visible to the naked eye and irritating enough to cause flashing and ulcers.

For related reading, see balanced nutrition, ich white spot disease treatment and community fish compatibility.

Identification

Flat, oval, translucent parasites crawling on skin and fins — often greenish. Fish flash, jump, and develop red irritation patches.

Large enough to see without magnification unlike ich — still confused with plant debris until they move.

Life Cycle

Argulus leaves fish to lay eggs on hard surfaces; juveniles must find hosts quickly. Empty tank fallow periods break the cycle alongside chemical treatment.

Eggs resist some water treatments — scrub hardscape in infected systems.

Treatment

Salt baths at species-tolerant concentrations kill mobile stages on some freshwater fish. Organophosphate dips where legal and appropriate for koi and goldfish.

Manual removal with tweezers for visible adults plus whole-tank treatment for juveniles.

Secondary Infections

Bite sites invite bacterial fin rot — monitor water quality and treat secondary infections if redness spreads.

Stressed fish stop eating; reduce competition and bright disturbance during recovery.

Prevention

Quarantine pond fish and plants. Argulus enters hobby tanks from outdoor sources more than indoor retail tropics.

Inspect new goldfish and koi closely — flat sides of fish show lice clearly.