Anchor Worms in Freshwater Fish

Causes, Treatment, and Prevention

anchor worm in fish

Dnatheist / Wikimedia Commons / CC by 2.0

Anchor worms are macroscopic parasites, meaning they can be seen by the naked eye. They are commonly found on koi and goldfish, but can be found on many freshwater fish species. The "worm" part extending out of the fish's skin into the water is actually the female reproductive structure. Treating these parasites can be challenging depending on your aquarium setup and the parasite load.

What Are Anchor Worms?

Anchor worms (Lernaea spp.) are not actually worms, but a crustacean copepod parasite that embeds into the fish’s skin and has a worm-like tail that sticks out of the skin. The head of the parasite is under the fish's skin and has an anchor-like appendage that holds it in the skin. These parasites have multiple non-parasitic stages that live in the water. Once a male mates with a female, she attaches to a fish to mature into a reproductively-active adult. These parasites get the name "worm" from the extending female reproductive structure. Juvenile anchor worms will be free-swimming in your aquarium, but will not be a nuisance for your fish.

The Lernaea genus of copepods infects most freshwater fish. They are commonly found on goldfish and koi. There are other similar genera of copepods that infect other freshwater and marine fish.

Symptoms of Anchor Worms in Fish

Anchor worms are one of the macroscopic freshwater fish parasites that are visible to the naked eye. Here are the obvious signs of anchor worms.


  • Visible worms on scales
  • Oral cavity problems
  • Skin patches and red lesions

Visible Worms on Scales

You will see the female reproductive structures that resemble short white worms sticking out from behind scales. The end of the white “worm” may have one or two egg sacs attached to it. These sacs contain their eggs and drop off into the aquarium to hatch.

Oral Cavity Problems

Anchor worms can also be found in the oral cavity of your fish. It will give the fish's mouth a "baleen" appearance, meaning that it looks like the long teeth of baleen whales.

Skin Patches and Red Lesions

Worms that have fallen off of your fish may leave behind skin patches of hemorrhage or fibrosis. Bacteria can infect the site of the anchor worm attachment, causing red lesions on the fish’s skin.

Causes of Anchor Worms

Anchor worms are contagious among fish and occur when a new fish is added to an aquarium carrying juvenile anchor worms or a reproductively-active female in their skin. Skipping proper quarantine makes this parasite spread very rapidly. A single female anchor worm can produce hundreds of larvae every two weeks for up to 16 weeks in a 77 degrees Fahrenheit aquarium.

Anchor worm juveniles can also be spread with the introduction of live plants. Although they may not be on the plants themselves, free-swimming juveniles may be in the water surrounding the plants. Aquatic plants can bring many bacteria and parasites into your aquarium if not properly quarantined. Like your new fish, quarantining plants in a plant-only system will break the parasite life cycle, since there are no fish to host them. This only applies to plants kept with fish. If your new plants have never been kept with any fish, they will be free from fish diseases. Though they may bring over some invertebrate pests, such as snails.

Diagnosing Anchor Worms in Fish

Since anchor worms are external parasites that attach themselves to the fish, you or your veterinarian can make a visual diagnosis. Oftentimes you will spot what looks like one or more slivers or loose threads dangling from various parts of your fish that you haven't seen before. However, anchor worms can be confused with what looks like algae or plant debris sticking out of the fish. A vet can use a microscope to confirm an infestation.


Anchor worms should be treated because they can proliferate and damage the gills, making it difficult for the fish to breathe. Once anchor worms are present on your fish, it is very tempting to simply pull them off, but resist the urge to do so. Anchor worms need to be removed correctly with a fish under sedation by your veterinarian. They will need to remove the entire parasite including the feeding end from under the fish’s skin. Depending on the level of infestation, sedation makes the process less stressful for the fish and allows the veterinarian to work more effectively without a squirming fish.

Once the mature females have been removed, you may still have a microscopic problem: the juvenile stages. Over-the-counter "anchor worm" treatments are usually fairly effective against the juvenile stages, but they will not kill the adults. Another way of killing the free-swimming juvenile stage in the aquarium can be accomplished by removing your substrate and decor and running your water through a UV light.

Treating anchor worms with organophosphates or diflubenzuron (dimilin) is effective, but needs to be undertaken with severe caution. Only use veterinary-approved products, keep them away from your other pets and children, and make sure to wear proper protection (i.e., gloves) when handling medications.

Prognosis for Fish With Anchor Worms

The sites of adult attachment may also develop secondary bacterial infections. Monitor these sites carefully after the adults have been removed. They may require treatment with antibiotics, depending on their site and severity. Antibiotics should not be purchased over-the-counter and should only be used when prescribed by a veterinarian. The best method of deterring these secondary bacterial infections is good water quality.

How to Prevent Anchor Worms

Anchor worms are highly contagious between fish. The best method of preventing anchor worms is properly quarantining any new additions to your aquarium. If you see any anchor worms present in a tank of fish you are looking to purchase, assume they are all infested. Remember, juvenile larvae are microscopic and may be present without you realizing it until it's too late. By properly quarantining your new fish, you will prevent the spread of this problem and other fish diseases to the main aquarium.