Argulus species are branchiuran crustaceans, and unlike most fish parasites they can detach, swim and reattach — including to a different fish, and including after being disturbed. Adults are visible as translucent or greenish discs that shift position on the host. The feeding lesions are the significant pathology: each is a puncture into the skin, and in numbers they cause serious irritation, blood loss and a series of entry points for bacterial infection. They are predominantly a pond and coldwater problem in the same way anchor worm is.
Short answer
Fish lice are flattened, disc-shaped crustaceans a few millimetres across that attach to the skin with suckers and move about on the fish. They are visible to the naked eye, which makes them one of the few aquarium parasites that can honestly be identified by looking. They feed by piercing the skin, leaving wounds that commonly become infected.
Recognisable by eye
The appearance is specific enough that a careful keeper can reach a working identification from the tank, though confirmation still improves the odds.
Water quality is the cause, the contributing factor or the deciding variable in the great majority of aquarium health problems. These steps come before any treatment decision, and for several conditions they are the entire treatment.
Every entry names the observation that separates it from this record. Work through these before deciding what you are treating — the most expensive mistake in fish health is confident treatment of the wrong thing.
How to tell it apart: Watch for two minutes. Anything that changes position on the fish is an animal; anything that does not is part of the fish or a lesion on it.
How to tell it apart: Anchor worm is a fixed linear thread emerging from the skin. Fish lice are flat discs that move about on the surface.
How to tell it apart: Leeches are elongated and extend and contract as they move; fish lice keep their flattened disc shape. Both are visible, and both arrive by the same routes.
Each entry states the mechanism, not just the association — an association without a mechanism is not something you can act on.
AquariumHQ names classes of response and never doses. Product concentrations differ between markets and formulations, several actives are temperature- and pH-dependent, and the invertebrate and plant tolerances that decide whether the rest of the tank survives are not printed on the label. Where a category puts a chemical in the water, what it can damage is stated with it.
Individual adults can be removed with forceps from a fish held safely in water. This gives immediate relief and reduces the breeding population, but does not address eggs already laid on tank surfaces.
Products active against parasitic crustaceans address the free-swimming and juvenile stages. The course has to extend across the egg-hatching interval, which at pond temperatures can be long.
What it can damage: These compounds are lethal to shrimp, crayfish and other ornamental crustaceans, and several affect snails and some insects. They must never be used where ornamental invertebrates are wanted.
Feeding lesions are frequently secondarily infected, and in a heavy infestation the cumulative skin damage is what actually threatens the fish.
What it can damage: Antibacterial products suppress nitrifying bacteria and can cause an ammonia spike during treatment.
AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.
Published rather than hidden. A health page that lists no limitations has not had its limitations looked for.
Each claim is shown with what kind of support stands behind it and how strongly we hold it. Those are separate axes: a measurement held tentatively and an editorial position held firmly are different things.
Argulus species, branchiuran crustaceans that attach to the host surface with suckers and feed by piercing the skin
Last verified 2026-08-11
Eggs are deposited on hard surfaces in the environment rather than on the host, so removal of visible adults does not end an infestation
Last verified 2026-08-11