Kutu Ikan & Anchor Worm: Parasites on Koi Fish and Goldfish in Malaysia

Kutu ikan usually means one of two visible parasites: the fish louse (Argulus), a flat, see-through disc about the size of a lentil that crawls over the skin, or the anchor worm (Lernaea), a thin thread a few millimetres to about a centimetre long stuck into the fish with a red, swollen spot at the base. Both are crustaceans, both are common on koi fish and goldfish in Malaysian ponds, and both are treatable. The fix: remove the visible parasites with tweezers, clean the wound, treat the whole pond or tank with a suitable anti-parasite product, and repeat treatment to catch the eggs and young.

How to Identify Kutu Ikan

	Fish louse (Argulus)	Anchor worm (Lernaea)
Looks like	Flat, round, translucent greenish or brownish disc	Thin white or greenish thread, often with two small egg sacs at the end
Size	Roughly 5–10 mm	A few mm to about 1 cm
Moves?	Yes, crawls and can swim off	No, stays fixed with its head buried in the fish
Where on the fish	Anywhere, often near fins and under the body	Often near fins, the vent and the base of the tail
Damage	Small red spots, irritation, possible infection	Red, swollen ulcer at the attachment point

Other signs of both:

Fish rubbing or flashing against walls and rocks
Jumping, restlessness, clamped fins
Red spots, sores and secondary infections
Reduced appetite

Both parasites are big enough to see. Net the fish into a shallow white container in good light and look closely.

Where Do They Come From?

New fish added without quarantine (the most common source)
Live foods or plants collected from ponds, drains or rivers
Wild fish caught locally and added to a pond
Equipment shared between ponds

Malaysia's warm water lets both parasites complete their life cycles fast, so a few lice can become an infestation within weeks.

Step 1: Remove Visible Parasites

For larger fish like koi and goldfish:

Prepare a clean container with pond or tank water.
Net the fish gently. A wet towel over the fish's eyes helps keep it calm.
Using fine tweezers, grip the parasite as close to the fish's body as possible.
For anchor worm, pull out slowly and steadily so the head doesn't break off inside.
For fish lice, pick them off; they come off more easily.
Dab the wound with an aquarium-safe antiseptic, such as povidone iodine on a cotton bud, avoiding the eyes and gills.
Return the fish quickly. Keep it out of water for as little time as possible.

If you're not confident, ask an experienced koi keeper or fish vet to help with the first fish.

Step 2: Treat the Whole Pond or Tank

Removing the adults you can see doesn't solve the problem. There are always eggs and young parasites in the water.

Use an anti-parasite product labelled for fish lice and anchor worm. Many keepers in Asia use products based on ingredients such as diflubenzuron, which interferes with how crustaceans moult, or other labelled anti-parasite treatments. Read the active ingredient and follow the label.
Browse pest removers and fish medication for options, and ask the seller whether a product treats argulus and lernaea.
Calculate the real water volume (L × W × average depth in cm ÷ 1,000 = litres) before dosing.
Repeat as the label instructs. Most treatments need two or more doses spaced a week or two apart to catch new hatchlings.

Important warnings

These treatments target crustaceans, so they will kill shrimp and may harm other invertebrates. Remove shrimp and crabs first.
Switch off UV and remove activated carbon if the label says so.
Keep strong aeration during treatment.
Don't combine treatments without checking compatibility.

Step 3: Clean Up and Prevent Infection

Wounds from anchor worms often become infected with bacteria.

Keep water very clean during and after treatment.
A low dose of garam ikan (aquarium or pond salt) can support healing and reduce stress for koi and goldfish.
Watch the wounds for spreading redness or ulcers, and treat as described in our ubat ikan luka guide.

Step 4: Break the Cycle

Fish lice lay eggs on hard surfaces like rocks, pond walls and pipes. Scrubbing surfaces during a water change helps.
Anchor worm larvae live in the water before attaching. Repeated treatment catches them.
Continue watching for at least a month after the last parasite.

Prevention

Quarantine all new koi and goldfish for 3–4 weeks. Inspect them closely. Our quarantine tank guide explains the setup.
Don't add wild-caught fish from rivers, drains or sawah to your pond.
Avoid live food from unknown ponds. Buy cultured live food or frozen food instead.
Rinse new plants and quarantine them.
Use separate nets for each pond or tank.
Inspect fish closely at feeding time; parasites are easiest to beat early.

Kutu Ikan Koi vs Other Problems

What you see	Could be	How to tell
Moving flat disc on the skin	Fish louse	It crawls and has a disc shape
Fixed thread with red base	Anchor worm	Doesn't move; may have egg sacs
Tiny white dots like salt	White spot (ich)	Many tiny dots, no thread or disc
Rubbing but no visible parasite	Flukes or other microscopic parasites	Needs closer inspection
Red streaks, no parasite	Water quality or bacteria	Test water

For other koi problems, see our penyakit ikan koi guide.

Frequently Asked Questions

Macam mana nak buang kutu ikan pada ikan koi? Tangkap ikan dengan berhati-hati, cabut kutu menggunakan penyepit halus sedekat mungkin dengan badan ikan, dan sapu luka dengan antiseptik yang selamat untuk ikan. Selepas itu, rawat seluruh kolam dengan ubat anti parasit dan ulang rawatan mengikut label.

Kutu ikan datang dari mana? Biasanya dibawa oleh ikan baharu yang tidak dikuarantin, makanan hidup atau pokok dari kolam dan longkang, atau ikan liar yang dimasukkan ke dalam kolam. Kuarantin ikan baharu selama 3 hingga 4 minggu.

Is anchor worm actually a worm? No. Despite the name, Lernaea is a crustacean (a copepod). Only the female attaches to fish.

Will kutu ikan treatment kill my shrimp? Very likely. Products that target fish lice and anchor worm affect crustaceans, which includes shrimp. Remove them first.

Can fish lice spread to humans? No. Fish lice and anchor worms are fish parasites and don't infect people. Wash your hands after handling fish anyway.

Get rid of parasites for good. Browse koi fish, pest removers and fish medication from Malaysian sellers on FishyHub.