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Identifying and Curing Anchor Worms in Freshwater Fish

Anchor worms are one of the more common freshwater fish parasites out there. They can show up seemingly out of nowhere. And once they arrive it can be difficult to get rid of them because many traditional medications don’t work so well on them. What are anchor worms and how can we cure them in our fish?

What Are Anchor Worms
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What are Anchor Worms?

Parasites hold special horror for us humans, especially people who grow up in first world countries where such infections are relatively rare. Well cared for pets typically have few to no parasites as they receive good quality food, medications, and are kept in clean living conditions.

But sometimes parasites can still find their way into our fish tanks. Most of these tend to be either completely internal, such as many worms and flukes, to so tiny that they don’t freak us out. Ich (Ichthyophthirius multifiliis in freshwater tanks and Cryptocaryon irritans in saltwater tanks) is a parasitic protozoan infection but the spots are too tiny to look like much to us.

Anchor worms, on the other hand, are just large enough to look truly disturbing. They protrude grotesquely from the skin of their host, catching the current and fueling our imaginations. But what are anchor worms, anyway?

Believe it or not, anchor worms are not worms at all, they just happen to look like them! Anchor worms are parasitic copepods of the genus Lernaea. All copepods are actually a type of crustacean, so they are more closely related to shrimp, crabs, and lobsters.

Many types of copepods are even an important source of food for marine fish and corals that eat zooplankton. So they can be found in specialty aquarium pet stores for sale fairly easily! Anchor worms are not edible, unfortunately. In fact, they find your fish to be rather delicious!

What Causes Anchor Worms in a Fish Tank?

It is especially common to find anchor worms on fish kept outdoors where there is more contact with the wider world. Ponds are a great habitat for a goldfish anchor worm to live out its life. They can find their way into your pond from a frog, bird, or other local wildlife transporting the eggs. You also find anchor worms on koi fish for this reason.

Live plants are also a source of anchor worms. Not adult worms; rather, the eggs that they release can find their way onto plants, which then get added to your pond or fish tank.

Can Humans Get Anchor Worms?

Fortunately, you have nothing to worry about here. Anchor worms do not infect humans; in fact they would find it impossible for two reasons. For one, these are aquatic organisms and have a life cycle revolving around the water. Since we are land animals it is not possible for them to complete their life cycle.

“But what if it gets inside of you?” you might be thinking? Assuming you were to do something silly like swallow anchor worm eggs or eat one (yuck), you still likely have nothing to worry about.

Anchor worms are specialized to parasitize fish. This means that their biology, methods of attaching to skin, and ways of defeating a host’s immune system rely on the animal being a fish. Not even aquatic birds, mammals, or invertebrates can catch anchor worms; parasites tend to be very specific to a particular host. So humans are not on their menu.

But a goldfish anchor worm or anchor worms on betta fish are pretty common. Why are anchor worms a problem for pet fish?

What Does an Anchor Worm Do?

Anchor worms are problems for aquarium fish because they are very large parasites. The head of the anchor worm is not the part sticking out of the fish; it is actually buried deep within the skin and muscle tissue.

Once inside the anchor worm feeds on blood and other body fluids. While they are pretty easy to see, a hidden anchor worm on fish can sometimes call your attention if you see a fish trying to scrape it off against rocks or driftwood. When the anchor worm feeds it causes the fish pain and irritation but it can’t be removed since fish don’t have hands or fingers to grasp it.

The “worm” portion is actually a stalk where the female anchor worm produces its eggs. The twin egg sack streams out from the body of the worm and over time it will release hundreds of eggs into your goldfish pond or aquarium. Once this process starts you will likely start finding more and more anchor worms on your fish.

What Causes Anchor Worms in a Fish Tank
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How to Treat Anchor Worms on Fish

Anchor worms are a little difficult to treat with medications at times. But they are not too difficult to remove by hand if you have a large aquarium or pond fish.

Physical Removal Anchor Worm Treatment

If you only see a handful of anchor worms on a fish then you may be able to remove them by hand. I recommend this for new fish that happen to have anchor worms; by identifying them before they enter your tank you can prevent an infestation of your other fish before it begins.

How to Remove Anchor Worms from Goldfish

To remove anchor worms from goldfish you need to first catch the fish in a net. With wet hands, you will then restrain the fish while using tweezers to grasp the anchor worm at its base. Pull upwards gently until the worm comes free of the flesh of the fish.

If your goldfish has several anchor worms then you will need to take a break every few seconds for the fish to breathe, placing it back into a small container of aquarium water. Once you have removed all of the anchor worms from its body, you can place it back into the fish tank or pond.

Be certain to check over the fish fully since anchor worms can attach almost anywhere, including the fin membranes, where they are almost invisible. If you have outdoor pond fish then place them in a clear container where you can inspect them carefully for anchor worms.

Anchor worm treatment involving physically removing the worm should only be done for medium sized fish or larger (5-6+ inches). So if you see an arowana anchor worm or goldfish anchor worm, it is safe to remove them carefully.

But I would not physically remove anchor worms on betta fish. A betta is just too small and the process would be very traumatic for it. A small piece of flesh and skin often comes with the anchor worm and a betta does not have much to lose. For small fish or ponds and aquariums that already have breeding anchor worms, we need to rely on medication instead.

Treat the tank you return the fish to with API Stress Coat Water Conditioner! This product uses aloe vera to stimulate their skin to begin producing healing mucus. This is like a scar, sealing the wound left behind and helping to jump-start the mending process.

Anchor Worm Treatment with Medications

Antiparasite medications are very effective on anchor worms in their larval (free-swimming) stage. But they don’t do much to the adults or the eggs. This can complicate treating them since the adults are free to continue sucking blood from your fish in the meantime.

Potassium permanganate and formalin dips are often effective in killing adult anchor worms. Just be certain to follow the directions to the letter because they can have side effects like drastically reducing oxygen levels in the water.

Many medications are also removed by the activated carbon in your power filter system. So I always treat sick fish in a quarantine tank for these reasons. Plus you need less medicine for a small quarantine tank versus the entire system.

But what if you aren’t treating just one fish because you know the entire tank or pond is infected by larvae? Then you have no choice but to dose the entire system. Be aware that many other animals can be sensitive to medications. Scaleless fish like Dojo Loaches are a prime example; they tend to soak up much more medicine, which can be fatal to them. Invertebrates like snails and shrimp can also be killed by medications and should be removed before any system-wide treatment begins.

Preventing Anchor Worm Infestations

The best weapon against having to perform anchor worm treatment is to find them before they ever enter your system. Carefully inspect any new fish, especially outdoor pond fish, for anchor worms, which look like small but stringy skin tags.

It helps to buy your aquarium plants from trusted sources as well that are known to be free of anchor worm eggs. Live plant aquascapers these days often buy live plant tissue cultures for this reason: the plants are grown in entirely sterile conditions so you won’t ever find any snails, flukes, or anchor worms in your tanks!

Frequently Asked Questions About Anchor Worms

How Do You Get Rid of Anchor Worms?

Anchor worms are a pretty persistent parasite once they begin breeding in your fish tank or pond. If you just find a few on a fish removing them by hand is easy to do. But if they are breeding then the entire system needs to be treated with a potassium permanganate or other anti parasite remedy to kill the larvae. This may take a while since the medication does not always work on adults or anchor worm eggs.

What Causes Anchor Worm?

Anchor worms are not actually worms at all! They are parasitic copepods of the genus Lernaea, making them crustaceans rather than worms. Therefore, anchor worms are more closely related to crabs, lobsters, and shrimp!

Can Anchor Worms Harm Humans?

While they do look gross, an anchor worm has no ability to infect humans. They depend on both fish and water to complete their life cycle. Nearly all parasites are highly specialized to their hosts and anchor worms are no exception.

What Does an Anchor Worm Do?

Anchor worms feed on blood and bodily fluids. They burrow into the flesh of their host fish and feed, which causes pain, irritation and weakness. Enough anchor worms can even cause fatal blood loss but this is very rare except in small fish with heavy infestations.

Jason Roberts
About Jason Roberts
Jason is an aquarium fanatic that has been a fish hobbyist for almost three decades.