flukes in koi fish
skin flukes on koi fish

Flukes - Worms

Flukes in koi fish, also known as fish lice or monogenetic trematodes, are a type of parasitic flatworm disease that can infect koi fish and other freshwater fish. These parasites attach themselves to the fish’s skin and gills, causing irritation, inflammation, and in severe cases, death.

Symptoms of Flukes


Symptoms of flukes in koi fish include scratching, flashing (rubbing against objects in the tank or pond), and thickened or discolored skin. The fish may also have difficulty breathing and may develop red or white spots on their skin. In severe cases, the fish may become lethargic and stop eating.

It is important for koi fish owners to be aware of the symptoms of flukes, as well as the causes and treatments, in order to protect their fish and maintain a healthy pond or tank environment. The most common symptoms of flukes in koi fish include:


  • Scratching: Fish will often scratch their skin against objects in the tank or pond, in an attempt to remove the parasites.
  • Flashing: Fish may also flash (rub against objects in the tank or pond) as a result of the irritation caused by the parasites.
  • Thickened or discolored skin: As the parasites attach themselves to the fish’s skin, it can become thickened and discolored, particularly around the gills and fins.
  • Difficulty breathing: As flukes attach to the gills, it can inhibit the fish’s ability to breathe properly, causing them to gasp for air at the surface of the water.
  • Red or white spots on the skin: In severe cases, fish may develop red or white spots on their skin as a result of the parasitic infection.
  • Lethargy and loss of appetite: Affected fish may become lethargic, stop eating and have a general lack of energy.


Flukes in koi fish are usually caused by poor water quality, overcrowding, or introducing infected fish to a pond or tank. They are most commonly found in warmer waters but can occur in any climate. If a fish keeper sees any of the above symptoms, it’s important to take the fish to a veterinarian or fish expert immediately, in order to confirm the diagnosis and begin treatment as soon as possible.




Once the diagnosis has been made, the next step is to treat the infection with antiparasitic medications, such as praziquantel or metronidazole. These medications are typically administered in the form of a water-based treatment, which is added to the fish’s environment. It’s important to follow the manufacturer’s instructions and to use the medication for the entire treatment period.

In addition to the medication, it’s important to maintain good water quality during the treatment period. This includes checking and adjusting pH, temperature, and dissolved oxygen levels, as well as performing regular water changes to remove any parasites and debris. Also, it’s a good idea to keep an eye on the fish and check for any signs of stress or lethargy, as this can be a side effect of the treatment.

It’s also important to understand that flukes in koi fish can return after treatment and it can be hard to completely eradicate the parasite. It’s important to check for them regularly and if symptoms appear to repeat the treatment or consult a veterinarian or fish expert.


Preventing Flukes in Koi Fish


Preventing flukes in koi fish is crucial in order to protect the fish and maintain a healthy pond or tank environment. Here are some ways to prevent flukes in koi fish:

  • Water Quality: Regularly check and maintain pH, temperature, and dissolved oxygen levels in the koi’s environment. Also, perform regular water changes to keep the water clean and free of harmful bacteria and parasites.
  • Diet: Feed your koi a high-quality diet that is appropriate for their size and species. Overfeeding or feeding the wrong types of food can lead to poor health and a weakened immune system.
  • Quarantine new fish: Before introducing new fish to your pond, quarantine them for at least two weeks to ensure they are not carrying any diseases. This will also give you an opportunity to observe the new fish and make sure they are healthy before they join the main population.
  • Stress management: Minimize stress on the fish by providing a peaceful environment and avoiding overcrowding. Also, be sure to avoid making too many changes to the pond or tank environment and to avoid over-handling.
  • Proper Maintenance: Be sure to maintain filters and other equipment to keep the water clean and free of debris. Also, keep an eye out for signs of disease or distress in the fish, and address any issues promptly.
  • Regularly check for parasites: Regularly check your koi fish for signs of flukes and other parasites and take appropriate action if they are found.
  • Use antiparasitic medications: Regularly use medications, such as praziquantel or metronidazole, to kill flukes and other parasites.


By following these steps, fish keepers can reduce the risk of flukes in their koi fish and ensure a healthy and happy population. It’s important to be proactive and take preventative measures as soon as possible, rather than waiting for an outbreak of flukes to occur. By keeping an eye on water quality, diet, quarantine of new fish, stress management, maintenance and regular checkups, fish keepers can effectively prevent flukes in their koi fish.