“Fish lice” usually refers to Argulus, a group of parasitic crustaceans that attach to the skin of freshwater fish. They are more common in ponds and outdoor systems, but they can arrive in aquariums on new fish or plants. They are visible with the naked eye, which makes them easier to confirm than many other parasites.
What fish lice are
Argulus are flattened, roughly disc-shaped parasites, often a few millimeters across, and are semi-transparent or greenish-brown. They have sucker-like structures to hold onto the host and a stinging mouthpart used to feed. Unlike many microscopic parasites, they can swim off a fish and move between hosts.
Symptoms
Signs that a fish may have lice include:
- Flashing, rubbing, or scratching against objects
- Restlessness, darting, or jumping
- Visible small, flat, moving shapes on the body or fins
- Red patches, small sores, or bleeding where parasites feed
- Clamped fins and reduced appetite
- Secondary infections, such as fungus or bacterial infection, at bite sites
Heavy infestations can weaken fish significantly, particularly smaller or younger fish.
Fish lice vs similar parasites
- Anchor worms (Lernaea) look like thin threads stuck into the skin, often with a small tuft at the end. Lice are flat and sit on the surface.
- Fish flukes are microscopic and are not visible without a microscope.
- Ich appears as numerous tiny white dots. See how to treat ich in fish.
- Sea lice are a different group of copepods that affect marine and farmed fish, such as salmon.
Removal
Because lice can be seen, physical removal can be effective for small numbers:
- Net the fish gently and keep it wet, or use a damp towel.
- Use blunt tweezers to remove each parasite. Take care not to damage the skin.
- Treat the wound area as advised by a vet or according to a suitable product.
- Return the fish to clean water, ideally a hospital tank.
Handling stresses fish and can damage their protective slime coat. For valuable fish such as koi, ask an aquatic vet or experienced koi keeper to help.
Treating the tank or pond
Lice lay eggs on hard surfaces like rocks, glass, and plants, and the eggs are not affected by many treatments. Larvae hatch and seek a host. Therefore, a typical plan includes:
- Removing visible adults from fish
- Using an anti-parasite treatment sold for fish lice or Argulus, following the label exactly
- Repeating treatment as the label directs to catch newly hatched larvae
- Removing and cleaning hard surfaces where possible
Many products harm invertebrates, some fish species, or biological filtration. Check the label and consider vet advice before treating a mixed tank.
Can humans get fish lice?
Fish lice are adapted to fish. They are not known to live on people, but they may nip if handled and some hobbyists report irritation. Wash your hands after handling infected fish or tank water.
Do sea lice go away on their own? How long do fish lice live?
Without treatment or intervention, lice usually persist while fish hosts are present. Lifespan depends on species and temperature. Argulus can complete their life cycle over weeks to months, and eggs can overwinter in some conditions. Warmer water generally speeds up their cycle. If the term “sea lice” refers to marine parasites on salmon or other ocean fish, those are a separate issue and not typically a home aquarium problem.
Prevention
- Quarantine new fish for several weeks and inspect them closely.
- Avoid adding wild-collected fish, plants, or water to ponds without caution.
- Inspect plants and decor from outdoor sources.
- Keep water quality high so fish can better withstand parasites.
After removal, bite wounds may become infected. See our guides to fish injury treatment and pond fish fungus treatment if sores develop.