How Do You Know If Your Koi Fish Has Parasites?

Koi fish are quite resilient once they become accustomed to their habitat. However, certain factors could cause your koi to become sick and infected with parasites, such as inadequate water quality. It is vital for the health of your koi that you perform regular parasite checks.

To know if your koi has parasites, you will need to check slime samples scraped from their scales under a microscope. Some parasites can be seen without using a microscope – you can spot redness, large worms, patches of film on the fish’s body and lethargy or sluggishness are all visible signs of parasites. 

It is essential to check your fish regularly for parasites, as some types can be highly contagious, and if you do not catch them quickly, most of your koi could die before receiving treatment. There are many ways to check and treat your koi fish yourself, but I would recommend calling an expert so that your koi can receive the best treatment possible.

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How To Tell If Your Koi Has Parasites

Unfortunately, parasites can be pretty common in fishponds and should always be taken seriously as untreated koi can become very ill and possibly die. As there are many kinds of parasites, it is essential to diagnose your koi as quickly as possible to receive the proper treatment. 

Internal Parasites In Koi Fish

Internal parasites make their home within the body of a koi fish, such as their digestive tract or the actual tissue of your koi. Due to this, it is harder to diagnose koi with internal parasites as the symptoms are less noticeable to the naked eye. 

Signs of internal parasites in your koi fish include a slower growth rate, a swollen belly, intestinal inflammation, a lower reproductive rate, muscle loss, and sluggishness or lethargy of the fish. This is when they stick to the bottom of your pond, have noticeably less energy, and might not eat as much as they usually would.

Suppose you notice any of these signs in your koi fish. In that case, it is vital to seek out professional help right away, as these could also be symptoms of bacterial infections, tumors and organ failure, or poor pond water quality. 

To diagnose and treat your koi with internal parasites, a veterinarian will examine the feces of your koi under a microscope to diagnose parasites within the digestive tract of the fish. For parasites that reside in the tissue of your fish, a surgical biopsy of the necrotic tissue may be needed. 

A treatment plan will be formed by your veterinarian, depending on the type of internal parasite they were able to diagnose. 

External Parasites In Koi Fish

External parasites infecting your koi will make their home on the surface of your fish’s skin or within the gill filaments of your koi. The signs and symptoms of external parasites are more visible and will be easier to diagnose than internal parasites.

The signs of external parasites in your koi will be anything from pale gills, respiratory distress, increased mucus production, which can make the scales appear blueish or cloudy, clamped fins, redness, sores, or ulcers on the skin, and secondary bacterial infections.

It is vital to call a professional immediately if you notice any of these signs in your koi, as they can also be symptoms of poor water quality, trauma, and bacterial infections. A professional will check for external parasites by scraping some skin off your koi and checking the sample under a microscope. 

The treatment for external parasites in koi depends on the type of parasite that has been diagnosed. There are many anti-parasite medications available that will help your koi. Some go directly into the pond water, while others can be applied directly to the koi or given to them in the form of food.

Different Types Of Koi Parasites And How To Treat Them

Multiple parasites could potentially infect your koi, which means there are different signs and symptoms you will need to look out for. 

Ich (Ichthyophthirius Multifilis)

Ichthyophthirius Multifilis is a microscopic protozoan parasite. The effects on your koi fish from being infected with this parasite are visible to the human eye. You will be able to spot hundreds of small white spots on the skin of your koi. 

Along with the white spots on the body and fins of your koi, you may notice the fish will begin scratching, it may have clamped fins, or you will be able to spot cloudy eyes and fins. 

There are chemical treatments available to rid your koi of these parasites. However, most treatments will need to be done more than once to ensure all the parasites are gone. Chemical treatments include Malachite Green with salt, Acriflavine with salt, and Parazoryne, 

Gill Flukes (Dactylogyrus)

Dactylogyrus is a microscopic worm that needs to be viewed under a microscope as it is not visible to the naked eye. Infected koi will show signs of irritation as they jump, flash, gasp at the pond’s surface, or rub against rocks and other objects in the pond. 

Gill flukes can be compared to fleas that infect cats and dogs, they are irritating, but one or two is not necessarily a problem. Flukes should be treated if they are discovered in high numbers or your koi are visibly agitated.

Anchor Worm (Lernaea)

Lernaea is a crustacean parasite that can be seen with the naked eye. You will be able to spot thread-like worms hanging from the skin of your koi, as well as small pimples. The anchor worm can grow up to 0.5 inches. 

Treatment for your koi when they are infected is essential, as if these worms are left on the koi for too long, they can cause secondary bacterial infections. You will need repeat chemical treatments for this parasite as the eggs only hatch when the temperature is ideal – therefore, you may have some worms left behind weeks or months after the initial treatment.

Chilodonella (Chilodonella Cyprini)

Chilodonella is a microscopic protozoan parasite that is highly deadly to koi fish. It is one of the most dangerous parasites, as in ponds with high bio-loads, it can cause mass fatalities. A professional will need to identify this parasite using a microscope. 

Symptoms include a white film that will cover most of the fish’s body, respiratory distress in the form of gasping or heavy breathing, and your koi will appear sluggish or lethargic and sit at the bottom of your pond.

The recommended treatment for Chilodonella includes Acriflavine with salt, Potassium Permanganate or Malachite Green, and Formalin. 

Fish Lice (Argulus) 

Argulus is a crustacean parasite that can be seen with the naked eye. Symptoms include visible distress in your fish, such as swimming erratically or flashing. There can also be signs of physical damage to the skin, such as inflammation, bleeding, and even a potential secondary bacterial infection.

Argulus needs to be diagnosed and treated as quickly as possible. The best treatment available to clear out these parasites is chemical treatments such as Lice Solve. However, as these parasites are visible to the eye, it is suggested that you try manually removing them first using tweezers before attempting any chemical options.


Some parasites can be visible to the human eye and are easy to diagnose and treat. Other parasites, such as internal parasites, are harder to spot. When checking if your koi has parasites, the main thing to look out for is redness, swelling, worms, and lethargy. 

I always recommend using professionals when dealing with infected koi, as some parasites can be hard to diagnose yourself. If there is a delay in providing the correct treatment, some of your koi may die.

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