Koi carp parasites: identify costia, trichodina, gyrodactylus, argulus and treat them

A koi that rubs against the bottom, jumps for no apparent reason, or keeps its fins clamped against its body: in the vast majority of cases, parasites are to blame. The usual reflex is to pour in a “broad-spectrum” antiparasitic treatment and hope for the best. The problem is that koi parasites are very different from one another, and a treatment that is effective against one may be useless against another. Identifying the parasite before treating saves time, spares the filter and fish, and prevents a relapse three weeks later. This guide helps you carry out this identification process and then treat correctly.

Why Parasites Explode in Spring

Most koi parasites are present at low levels in many ponds without causing any visible problems. They become troublesome when the balance shifts: weakened fish, degraded water, or excessive stocking density. Spring often brings these conditions together.

During winter, koi live at a slower pace, and so does their immune system. When the water rises above around ten degrees, several parasites quickly become active again, with some even thriving in cool water. The fish's defenses, however, do not regain their full effectiveness until the water is warmer, above around fifteen degrees. During this window of a few weeks, the advantage lies with the parasites. Add a filter that is restarting slowly and bodily reserves depleted by winter, and you have the period when we receive the most questions about koi rubbing themselves.

Before looking for a parasite, always test the water: ammonia, nitrites, or unstable pH can also irritate the skin and gills and cause rubbing that is very similar. Our article about koi rubbing themselves explains this first step in detail.

Visible parasites: argulus, leeches, lernaea

Three parasites can be seen with the naked eye, provided you look closely at a fish placed in a clear tank.

Argulus deserves special attention. Females lay their eggs on hard surfaces (walls, stones, pipes), and the young hatch over several weeks. A single treatment kills the adults present but not the eggs: treatment generally needs to be repeated according to the instructions to target subsequent generations, and the egg-laying surfaces should be cleaned. Bare-skinned koi, such as Doitsu, make it possible to detect these parasites very early.

Microscopic parasites: costia, trichodina, chilodonella

These three protozoa cannot be seen with the naked eye. They are suspected based on clinical signs and identified under a microscope from a mucus smear.

These protozoa have short life cycles and can multiply very quickly. A massive infestation of costia or chilodonella on the gills can kill fish within a few days: if you see lethargic koi breathing rapidly, do not delay performing a smear.

Gill and skin worms: dactylogyrus, gyrodactylus

Monogeneans are small flatworms, barely visible to the naked eye, equipped with hooks that they use to attach themselves to tissues. Two genera cause most of the problems:

• Gyrodactylus mainly lives on the skin and fins. It is viviparous: under a microscope, an already formed embryo can often be seen inside the worm. Its direct reproduction, without an egg, explains rapid infestations.
• Dactylogyrus mainly lives on the gills. It lays eggs that resist many treatments: treatment often needs to be repeated after hatching, according to the instructions and temperature.

Associated signs include rubbing, damaged fins, excessive mucus and, for gill worms, rapid breathing and slightly open opercula. Salt alone is not very effective against these worms: they require specific treatment.

Taking a smear: equipment and method

A mucus smear is the basic diagnostic tool. It requires minimal equipment and a little practice; many hobbyists do it themselves, while others prefer to entrust it to a professional.

Equipment: a microscope capable of magnifying approximately 40 to 100 times to spot parasites, then more to identify them; clean slides and coverslips; a pipette; pond water (never tap water, which can kill or deform parasites); a fish net and a container to immobilize the fish.

1. Capture the fish calmly and keep it in a shallow container or a wet sock. For large fish, sedation by a professional makes the procedure safer.
2. Collect the mucus by scraping very gently, in the direction of the scales, with the edge of a coverslip: on the flank, behind a pectoral fin, or near the base of a fin. A little mucus is enough; do not tear off any scales.
3. Place it on a slide with a drop of pond water, then cover with a coverslip.
4. Examine it immediately, first at low magnification to spot anything moving, then at higher magnification to identify it. Protozoa slow down and then die within a few minutes off the fish.
5. Note what you see: type of parasite, approximate number per field, sampling area.

Sampling from the gills is more delicate and generally reserved for professionals. A negative skin scrape does not rule out a problem with the gills: if respiratory signs predominate, request a complete examination.

Choose the treatment and calculate the dose

Once the parasite has been identified, choosing the treatment becomes straightforward: a product suited to that parasite and temperature, and compatible with the other pond inhabitants. Our article on medicines for koi carp reviews the main categories of treatments available.

Then there is the dose, the source of most failures. An underdosed treatment does not kill the parasites and selects the most resistant ones; an overdosed treatment can kill the fish. So everything starts with the pond's actual volume:

Add the volume of the filter and pipes if significant. Weigh or measure the product accurately, dilute it in a bucket of pond water, and distribute it over the entire surface with the filter running. Never combine two treatments without checking their compatibility, and observe the intervals indicated between applications. Our article on treating fish effectively covers these basic rules.

UV light, bacteria, and food during treatment

Treatment affects the entire pond, not just the parasites. A few adjustments help avoid collateral damage:

After treatment, perform a partial water change if the instructions recommend it, switch the UV back on, put the adsorbent media back in, and, if necessary, restart the filter with bacteria. Resume feeding gradually.

Prevent reinfestations

Successful treatment is only worthwhile if the conditions that encouraged the parasites have changed. The most effective measures:

• Systematic quarantine for new fish and plants from other ponds or the natural environment, which may carry eggs and leeches. Our article on quarantine salt baths describes a simple and useful procedure.
• Reasonable stocking density: the more fish there are, the more easily parasites spread from one to another.
• Healthy water: little organic matter, a well-maintained filter, and sludge removed regularly.
• Regular checks in spring and autumn, high-risk periods, with a scrape at the slightest doubt.
• Managing treatment cycles: for argulus and egg-laying worms, follow the treatment repeat schedule.
• High-quality food suited to the temperature, for fish in good condition.

A common case in consultations: a pond treated three times in one month with three different products, without a diagnosis, while the koi remained just as restless. A scrape revealed a heavy infestation of skin flukes, against which none of the products used was suitable. A single targeted treatment, correctly dosed and repeated at the right time, was enough. Identify first, treat second: it is faster, cheaper, and far less stressful for the fish.

Also read

For other bacterial and viral conditions: Koi carp diseases - How to treat them?.

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Frequently asked questions

— Why are my koi rubbing themselves against the bottom?

Most often due to skin or gill parasites, sometimes due to irritating water (ammonia, nitrites, unstable pH). Test the water first, then have a mucus smear examined to identify the parasite.

— How can you recognize carp lice (argulus)?

They are small, flat, translucent discs a few millimeters across that move over the skin and fins. They often leave red spots. They can be seen with the naked eye, especially on koi with light or bare skin.

— Can parasites be treated with salt?

Salt is effective against several skin protozoa at suitable concentrations and treatment durations, but has little effect on flatworms and none on crustaceans such as argulus. It must be dosed according to the actual volume and is not compatible with all plants.

— Should the UV lamp be switched off during treatment?

Often yes: UV can degrade certain active substances. Follow the product instructions, which specify whether the UV should be switched off and for how long.

In summary

• Visible parasites (argulus, leeches, lernaea) can be spotted with the naked eye; the others require a smear and microscope.
• Costia, trichodina, and chilodonella are skin protozoa; gyrodactylus and dactylogyrus are flatworms of the skin and gills.
• Spring is the high-risk season: parasites become active again before the fish's defenses do.
• Treat after identification, dose according to the actual volume, increase aeration, and manage UV, filtration, and feeding.
• Prevent relapse: quarantine, reasonable stocking density, clean water, and control of life cycles (argulus eggs, worms).

Last updated: October 3, 2026.