What causes ulcers in Koi and Goldfish?

What Causes Ulcers in Koi and Goldfish?

gill fluke under microscope
Both gill flukes (pictured) and skin flukes are common parasites which can cause ulceration in koi and goldfish.

As mentioned above, the most common cause of ulcers are injuries which become infected with particular strains of bacteria living in the pond. Both aeromonas and pseudomonas bacteria strains are the most common culprits, and koi will become more susceptible to infection if they’re living in poor conditions or are generally unhealthy.

Wounds can occur from parasites, such as flukes, or predator attacks, such as herons and cats. Gill and skin flukes are particularly notorious in bringing about ulcers, as unlike internal parasites (i.e. worms), they attach themselves externally and cause sores which are exposed to outer pathogens. Parasites are also very contagious, with their eggs laying dormant for several weeks before hatching, which leads to future infections in other fish. Parasites can be prevented by ensuring koi are in optimal health and water conditions are within safe parameters, but an active outbreak would still require treatment with specialised products to stop spreading.

Aeromonas hydrophila – a common bacteria found in ponds which can cause ulceration and infection.

Predators can also cause injury to koi which can lead to ulcers, especially if you have herons in your area. Even if you don’t spot a heron in your garden, you could still be having daily visits as they like to arrive at first light and before dusk. Stressed fish, nipped fins, and damaged scales are all signs of a predator stalking the pond.

Finally, poor water conditions or general malnourishment can eventually lead to ulcers appearing as fish become sick and stressed. Poor water quality leads to more harmful bacteria growth and substance imbalance, which can have an impact on a kois immune system. As well as this, feeding a low quality fish food without the correct ingredients can lead to a reduction in slime coat (their first line of defence), slower healing, and less of a immune response.

Causes of Ulcers (Summarized):

  • Increase in harmful pond bacteria
  • Parasite infections (flukes)
  • Predator attacks
  • Poor water quality & conditions
  • Lack of proper nutrition (comprised slime coat)
  • Poor “winterization” routine

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