Freshwater Fish with Anchor Worms

anchor worm in fish

Anchor worms are macroscopic parasites that are visible to the human eye. They're most typically found on koi and goldfish, but they're also found on a variety of other freshwater fish. The "worm" component of the fish's skin that extends into the water is the female reproductive structure. Depending on your aquarium arrangement and parasite load, treating these parasites might be difficult.

What Are Anchor Worms?

Anchor worms (Lernaea spp.) are crustacean copepod parasites that embed themselves in the skin of the fish and have a worm-like tail that hangs out. The parasite's head is hidden beneath the fish's skin and is held in place by an anchor-like attachment. Non-parasitic stages of these parasites can be found in the water. She adheres to a fish to grow into a reproductively active adult when a male mates with a female. The worm-like female reproductive structure gives these parasites their name. The juvenile anchor worms will swim freely in your aquarium and will not bother your fish.

Most freshwater fish are infected by the Lernaea copepod genus. Goldfish and koi are the most common hosts. Various freshwater and marine fish are infected by copepods from other genera.

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

The female reproductive structures, which resemble small white worms and protrude from beneath scales, may be seen. One or two egg sacs may be linked to the white "worm's" end. Their eggs are contained in these sacs, which they drop into the tank 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

When a new fish is introduced to an aquarium with young anchor worms or a reproductively active female under its skin, anchor worms become infectious. This parasite spreads quickly when strict quarantine is not followed. In a 77°F aquarium, a single female anchor worm may generate hundreds of larvae every two weeks for up to 16 weeks.

The introduction of living plants can help distribute anchor worm juveniles. Free-swimming juveniles may be present in the water around the plants, even if they are not on the plants themselves. If not adequately confined, aquatic plants can introduce a variety of germs and parasites into your aquarium. Because there are no fish to host parasites, quarantining plants in a plant-only environment will disrupt the parasite life cycle. This solely applies to fish-keeping plants. Your new plants will be free of fish infections if they have never been kept with any fish. They may, however, introduce such as snails.

Diagnosing Anchor Worms in Fish

Because anchor worms are external parasites that adhere to the fish, a visual diagnosis can be made by you or your veterinarian. Occasionally, you'll notice what appears to be one or more slivers or loose threads dangling from previously unseen sections of your fish. Anchor worms, on the other hand, might be mistaken for algae or plant detritus protruding from the fish. A microscope can be used by a veterinarian to confirm an infestation.


Because anchor worms can multiply and damage the gills, making it harder for the fish to breathe, they should be treated. It's tempting to just rip anchor worms off your fish after they've been discovered, but resist the desire. Anchor worms must be removed appropriately by your veterinarian while the fish is sedated. They'll have to get rid of the parasite completely, including the feeding end, from beneath the fish's skin. Sedation makes the treatment less uncomfortable for the fish and allows the veterinarian to operate more efficiently without a wriggling animal, depending on the extent of infestation.

You may still have a microscopic problem after the older females have been removed: the juvenile stages. Adults are not killed by over-the-counter "anchor worm" treatments, which are normally fairly successful against the juvenile stages. Removing your substrate and décor and passing your water under a UV lamp is another approach to eliminate the free-swimming juvenile stage in your aquarium.

The use of organophosphates or diflubenzuron (dimilin) to treat anchor worms is successful, but it must be done with extreme caution. When handling pharmaceuticals, only use veterinary-approved items, keep them away from other pets and children, and use protective protection (i.e., gloves).

Prognosis for Fish With Anchor Worms

Secondary bacterial infections can occur at adult attachment sites. After the adults have been removed, keep a close eye on these areas. Antibiotics may be required, depending on the location and severity of the infection. Antibiotics should not be bought over-the-counter and should only be used if a veterinarian has recommended them. Water quality is the most effective way to prevent subsequent bacterial infections.

How to Prevent Anchor Worms

Fish are particularly infectious when it comes to anchor worms. The easiest way to keep anchor worms at bay is to quarantine any new aquarium additions appropriately. If you notice any anchor worms in a tank of fish you're considering buying, presume the entire tank is contaminated. Remember that young larvae are minute, and they may be there without your knowledge until it's too late. You may avoid the transmission of this and other fish illnesses to the main tank by carefully quarantining your new fish.