Fish Lice
Argulus
High severity ⚠️ Contagious
visible disc-shaped crustacean parasites that clamp onto fish and cause intense flashing
Symptoms
Catching illness early gives the best chance of recovery, so check your fish daily. Visible flat, disc-shaped parasites (3-5 mm) crawling on the skin and fins, intense flashing and rubbing, reddened attachment points and restlessness. Act promptly if you see several signs together.
Causes
A crustacean parasite (Argulus) usually introduced with new fish, pond fish, plants or live food, that clamps onto fish and feeds on blood. The underlying trigger is almost always stress, poor water quality or a newly introduced animal.
Treatment
Remove visible lice with tweezers and dab wounds with antiseptic, then treat the whole tank with an anti-crustacean medication (e.g. containing diflubenzuron) to kill larvae over several weeks.
While treating, increase aeration, remove activated carbon from the filter, finish the full course even after symptoms fade, and keep the water immaculate. Marine treatments belong in a separate quarantine tank, never the reef.
Prevention
Quarantine new fish and pond plants and inspect arrivals carefully, as fish lice almost always arrive on new stock. The golden rules: quarantine new arrivals, never overstock or overfeed, avoid sudden swings, and keep up regular water changes.
FAQ
What is Fish Lice in fish?
How serious is Fish Lice?
How do I treat Fish Lice?
Can Fish Lice be prevented?
Will it spread to my other fish?
This information is general guidance, not veterinary advice. Consult an aquatic vet for serious cases.