Anchor Worm in Koi: Identification, Risks, and a Proven Treatment Approach
Anchor worm (Lernaea) is a less common but highly destructive parasite in koi ponds. This condition is covered as part of our Koi Diseases & Treatment Guide. In my experience, its presence is often linked to wildlife exposure — ponds frequented by birds, amphibians, or other animals have a higher risk of introduction.
Well-protected ponds see anchor worm far less often. However, when it does appear, it needs to be handled correctly and without delay.
What Anchor Worm Really Is
Despite the name, anchor worm is not a worm. It is a parasitic crustacean that physically embeds itself into the body of the fish. The parasite penetrates through the slime coat and skin, anchoring its head deep into tissue while part of its body remains visible.
Anchor worm can usually be seen with the naked eye, although it may be small in early stages.
Why Anchor Worm Is Dangerous
Anchor worm itself is rarely what kills the fish.
The real danger comes from what it causes:
Penetration of the slime coat
Damage to the skin barrier
Open wounds at the attachment site
This damage allows opportunistic bacteria to enter, often leading to ulcers and systemic bacterial infections. In most cases, it is the secondary bacterial infection — not the parasite — that ultimately kills the fish.
Signs of Anchor Worm Infestation
Visible parasite protruding from the skin or fins
Redness and swelling at attachment sites
Flashing or irritation
Loss of appetite
Ulcer formation around the wound
Any visible anchor worm should be treated promptly.
My Treatment Philosophy for Anchor Worm
Anchor worm is not a parasite you ignore, but it is also not something most koi keepers should fear.
I consider anchor worm a manageable parasite when approached correctly. The key is understanding that you must treat the entire system, not just the affected fish.
If an anchor worm infestation results in an ulcer:
Treat the entire pond for anchor worm first
Then address the ulcer as a separate bacterial issue
Treating the ulcer without eliminating anchor worm from the system guarantees recurrence.
Proven Anchor Worm Treatment Protocol
Whole-Pond Treatment
When anchor worm is identified, treat the entire pond system using:
Dose:
50 mL per 1,000 gallons
Maintain normal filtration and good aeration during treatment.
This interrupts the parasite’s life cycle and prevents reinfestation.
Repeat Treatment
Repeat the dose after 5 days
This step is critical. Anchor worm has multiple life stages, and a single treatment is not sufficient to eliminate all developing parasites.
Addressing Resulting Ulcers
If an ulcer develops as a result of anchor worm attachment:
Treat the pond for anchor worm first
Then follow a proper ulcer treatment protocol
Ulcer management is covered in detail in the ulcer treatment guide.
Prevention Through Proactive Management
Anchor worm is far easier to prevent than eliminate.
I recommend:
Treating for anchor worm twice per year
Once before winter, before water temperatures drop below 65°F
Once coming out of winter as water warms and wildlife activity increases
This proactive approach provides peace of mind and dramatically reduces the likelihood of ever dealing with anchor worm.
When parasite pressure is kept low, anchor worm — along with many other parasites — becomes a non-issue.
Final Thoughts
Anchor worm is destructive, but it is not mysterious or unbeatable.
When treated proactively and correctly:
The parasite is controlled
Secondary infections are avoided
Fish recover quickly
Long-term pond health is preserved
Proactive parasite management prevents most problems before they ever start.
