Is your koi flashing, clamping its fins, producing excess slime, isolating itself, or breathing heavily? Parasites may be responsible but these symptoms can also be caused by poor water quality, low oxygen, bacterial infections, or sudden environmental changes.

That's why we recommend diagnosing the problem before treating the pond.

A good process is:

Test the water → Observe the koi → Inspect the fish → Scrape and scope → Identify the parasite → Treat appropriately

1. Test Your Pond Water First

Before reaching for medication, test:

• Ammonia

• Nitrite

• Nitrate

• pH

• KH

• Water temperature

If possible, also check dissolved oxygen when fish are breathing heavily or during hot weather.

Water quality problems can produce many of the same symptoms as parasites. Poor conditions also stress koi and make them more vulnerable to disease.

Don't skip the water test just because the fish looks like it has parasites.

Click here to see our recommended Test Kit.

2. Observe Your Koi

Watch the pond before catching or disturbing the fish.

Look for:

• Flashing or rubbing

• Clamped fins

• Lethargy or isolation

• Loss of appetite

• Excessive mucus

• Rapid breathing

• Hanging near waterfalls or returns

• Gasping at the surface

• Redness, sores, or ulcers

• White spots

Also consider what's happening throughout the pond. Are multiple fish affected? Were new koi recently introduced? Did the temperature change? Was anything recently added to the water?

Remember: flashing, clamped fins, and excess mucus are symptoms not diagnoses.

3. Inspect the Koi

Carefully examine the affected koi for:

• Excessive slime coat

• Redness

• Missing or raised scales

• Ulcers

• White spots

• Fin damage

• Gill abnormalities

• Visible parasites

Pay particular attention to the gills when a koi is breathing rapidly or staying near highly oxygenated water.

Fish lice and adult anchor worms may be visible without a microscope. Most other common external koi parasites require microscopic examination.

4. Perform a Skin Scrape and Scope

A skin scrape can move you from suspecting a parasite to identifying one.

A small mucus sample is collected from the koi, placed on a microscope slide with water and examined while fresh.

Fresh samples are important because movement can help identify parasites.

Whenever possible, examine samples from more than one area of the fish. Parasites aren't always evenly distributed.

A negative scrape also doesn't guarantee that the koi is parasite-free—it only means you didn't find parasites in that particular sample.

Safety Note: Gill sampling is more invasive than collecting skin mucus. If you're not experienced handling koi or collecting gill samples, seek assistance from an experienced koi health professional or aquatic veterinarian.

Common Koi Parasites: What to Look For

Common external koi parasites. Actual appearance can vary depending on life stage, orientation, magnification, microscope settings, and sample quality.

Alt text: Common koi parasite identification guide showing skin flukes, gill flukes, Trichodina, Chilodonella, Costia, Ich, fish lice, and anchor worm.

Skin Flukes – Gyrodactylus

Skin flukes look like tiny transparent worms. Look for an elongated body and a hook-bearing attachment structure at the rear. You may even see a developing young fluke inside an adult.

Found on: Skin and fins
Movement: Stretching and contracting

Click here to see our recommended treatments for Skin Flukes.

Gill Flukes – Dactylogyrus

Gill flukes are also transparent, worm-like parasites with hooks used for attachment. They're particularly important to investigate when koi are experiencing respiratory problems.

Found on: Primarily the gills
Movement: Stretching and contracting

Click here to see our recommended treatments for Gill Flukes.

Trichodina

One of the easier parasites to recognize. Trichodina often looks like a tiny wheel or circular saw blade under the microscope.

Found on: Skin and gills
Movement: Gliding and spinning

Click here for our recommended treatment for Trichodina.

Chilodonella

Chilodonella is smaller and may appear flattened, oval, heart-shaped, or kidney-shaped depending on its orientation.

Found on: Skin, fins, and gills
Movement: Gliding

Click here for our recommended treatment for Chilodonella.

Costia – Ichthyobodo

Costia is extremely small and easy for beginners to miss. It may appear pear-shaped at sufficient magnification.

Found on: Skin, fins, and gills
Movement: Jerky, darting, or spiraling. Attached organisms can have a flickering appearance.

Microscope Tip: Don't stop after scanning at low magnification. Increase magnification and carefully look for smaller parasites such as Costia.

Click here for our recommended treatment for Costia.

Ich – Ichthyophthirius multifiliis

Ich is much larger than Costia. Mature organisms are round or oval, covered in cilia, and may show the characteristic horseshoe-shaped macronucleus.

Found on: Skin, fins, and gills
Movement: Rolling or rotating

Don't rely solely on visible white spots. Ich can affect the gills without producing obvious external spots.

Click here for our recommended treatment for Ich.

Fish Lice – Argulus

Fish lice are flattened crustaceans that are generally large enough to see without a microscope.

Found on: Skin and fins
Movement: Crawls across the fish

Click here for our recommended treatment for Fish Lice.

Anchor Worm – Lernaea

Despite its name, anchor worm is a parasitic crustacean. Adult females may look like whitish or greenish threads protruding from the koi's skin, often with redness around the attachment point.

Found on: Skin and fins
Identification: Usually visible without a microscope

Click here for our recommended treatment for Anchor Worms.

Quick Koi Parasite Identification Chart

Parasite	Where to Look	Appearance	Movement
Skin Fluke	Skin & fins	Transparent worm with rear hooks	Stretches/contracts
Gill Fluke	Gills	Transparent worm with hooks	Stretches/contracts
Trichodina	Skin & gills	Circular wheel	Spins/glides
Chilodonella	Skin, fins & gills	Flattened oval/heart shape	Glides
Costia	Skin, fins & gills	Very small, pear-shaped	Jerky/flickering
Ich	Skin, fins & gills	Large, round ciliate	Rolls/rotates
Fish Lice	Skin & fins	Flat disc-like crustacean	Crawls
Anchor Worm	Skin & fins	Thread protruding from skin	Attached

Don't Treat Based on Symptoms Alone

Finding the right treatment starts with identifying the problem.

Different parasites respond to different medications, and unnecessary treatments can add additional stress to already compromised koi.

Before treating, confirm:

• What parasite you're targeting

• Your actual pond volume

• Correct medication and dosage

• Water temperature

• Whether UV, carbon, ozone, or other equipment needs to be adjusted

• Whether additional aeration is recommended

• Whether repeat treatments are necessary

Never combine medications unless the manufacturers specifically indicate they can be safely used together.

After treatment, continue monitoring your koi and water quality. When appropriate, perform another scrape and scope to see whether the treatment was successful.

The Bottom Line

When your koi starts acting sick, it's natural to want to treat immediately. But the fastest route to the right treatment is often taking the time to make the right diagnosis first.

Test the water. Observe the fish. Inspect the koi. Scrape and scope. Identify the problem. Then treat.

And remember:

Nothing good happens fast in a koi pond.

Make changes carefully, monitor your fish closely, and ask for help when you're unsure what you're seeing.

This guide is intended for educational purposes. Fish experiencing severe distress, unexplained mortality, significant wounds, or conditions that cannot be confidently identified should be evaluated by a qualified aquatic veterinarian or appropriately trained fish-health professional.