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905-715-2447 Winter Hours: By Appointment Only 3301 Sideroad 10, Bradford, ON Canada

Koi Disease Diagnosis and Treatment

Koi Disease Diagnosis and Treatment

Koi and goldfish are typically very hardy, robust fish and don’t often once become sick once they have settled into the pond. Occasionally, goldfish and koi disease occurs in ponds as fish fall prey to parasitic, bacterial or fungal attacks. The causes of fish diseases are varied and can range from a sudden drop in water temperature, predator attacks and spawning to name a few. No matter what the cause of the disease, one thing remains constant – the sooner you recognize and begin to treat the problem …the more likely you will be successful treating the disease. So how do you know if you have a sick fish? Sometimes the signs that a fish is sick are very subtle, such as one fish segregating itself away from other fish, or maybe not eating very much. As time passes and the disease gets worse, the symptoms become more obvious and may spread to other fish.

Koi Disease Diagnosis and Treatment - Parasites

Causes of Koi Disease

Koi and goldfish are very hardy, resilient fish. In most cases, stress is the cause of most diseases in fish. Stress? Yes, but the stress that causes fish to get sick is usually in the form of poor water quality, overcrowding or other environmental factors. If koi or goldfish are kept in ideal conditions, they rarely get sick because they are able to fight off possible attacks from parasites or bacteria.

  • Stress
  • Poor water quality
  • Introducing sick fish to your pond
  • Spawning
  • Overcrowding
  • Contamination (pesticides, fertilizer etc.)


Signs of Koi Disease

There are several ‘early warning signs’ that indicate that your fish may be under attack from parasites or bacteria. The first sign of a problem usually starts with one fish that segregates itself from the rest of the school, often hanging listlessly near the surface of the water. If this fish doesn’t eat, it’s a sure sign that something isn’t right. Recognizing the first signs of disease is important because it can be controlled much more easily if it is caught early. Depending on the situation, disease can spread to many or all fish in the pond relatively quickly. It’s always a good idea to take count of your fish at feeding time and also take a close look them while they are close to the surface.

  • Not eating
  • Fish segregating itself from others
  • Fins clamped close to body
  • Fish acting listless or lethargic
  • Gasping at surface of pond
  • Fish Sitting on bottom
  • Hanging near surface or near waterfall
  • Red streaks in fins
  • Ulcers (red sores)
  • White spots
  • Ragged fins
  • Fluffy cotton-like tufts



Koi Disease Identification

The only way to properly identify parasites on fish is take a skin scrape and look at it under a microscope. Most people don’t have microscopes, so the next best thing is to take a close look at your fish and try to compare the symptoms to the diagram below.

koi parasite disease diagnosis chart


Treatment of Koi Disease

If one of your koi or goldfish shows any of the symptoms listed above, then check the tables below to find the recommended medication and begin treatment as soon as possible. The first table deals with parasite symptoms and treatments, the second table deals with bacteria and fungal infections. If you have just one fish that is sick, you may want to set up a quarantine tub or hospital tank and treat only that fish. On the other hand, if you have several fish that are sick, treating the whole pond is advisable for best results.

Parasite SymptomsPossible CauseRecommended TreatmentOther Treatments
White spots on body and fins resembling salt, fish scratching, clamped fins, cloudy eyes and finsIch
(ichthyophthirius multifiliis). A microscopic protozoan parasite
Broad Spectrum Disease TreatmentAlso try Pond Fish Treatment
Fish gasp at surface, flash or jumpGill Flukes
(Dactylogyrus). A microscopic worm
PraziquantelPraziquantel Fluke medication
Thread-like worms hanging from skin , small pimplesAnchor Worm
(Lernaea) A crustacean parasite visible with the naked eye.
Physical Removaltweezers
White film covering most or all of the body, gasping or heavy breathing, fish are lethargic sitting on bottomChilodonella
(Chilodonella cyprini) A microscopic protozoan parasite
Broad Spectrum Disease TreatmentAlso try Pond Fish Treatment
Fish are lethargic, sitting on bottom, gasping or breathing heavily, flashing, small reddish lesionsTrichodina
(Tichodina sp) A microscopic protozoan parasite
Broad Spectrum Disease TreatmentAlso try Pond Fish Treatment
Patches of bluish/white film on body, small reddish lesionsCostia (Ichthyobodo necatrix) A microscopic parasite Broad Spectrum Disease TreatmentAlso try Pond Fish Treatment
Sluggishness, flashing, spider web lesions on skin, reddened fins, clamped finsBody Flukes (Gyrodactylus). A microscopic wormPraziquantelPraziquantel Fluke medication
Flashing, swimming erratically. Smalll clear disc-shaped up to 1 cm diameter can be seen with the naked eye.Fish Lice (Argulus) A crustacean posiible visible with the naked eye.Physical Removaltweezers
Large dark 'worms' attached to the fishes body are visible with the naked eyeLeeches (Piscicola sp). segmented worm. visible with the naked eye.Physical Removaltweezers

Bacterial / Fungal SymptomsPossible CauseRecommended TreatmentOther Treatments
Swollen abdomen, scales sticking outDropsy. Often caused by internal bacterial infectionWipeOutWipeOut-bacterial-medicationAlso try Medicated Koi Food
White cotton-like patches on fins, body, or mouth (sometimes also green in colour).Fungus (Saprolegnia sp.) PimafixPimafix-anti-funal-medicationAlso try KnockoutKnockOut-parasite-medication
Open sores with red edges.Ulcer - (Pseudomonas sp) Bacterial InfectionTricide NeoTricide-Neo-bacterial-ulcer-dip-medicationSee our How to Treat Ulcers Article
Eye swollenPop Eye - (Pseudomonas sp) Bacterial InfectionWipeOutWipeOut-bacterial-medicationAlso try Medicated Koi Food or MediMax
Red streaks in body and fins, or redness in the body.Bacterial Hemorrhagic Septicemia (Aeromonas sp.)WipeOutWipeOut-bacterial-medicationAlso try Medicated Koi Food or MediMax
Damaged , shredded fins or tailFin Rot (Pseudomonas sp) Bacterial InfectionWipeOutWipeOut-bacterial-medicationAlso try Medicated Koi Food or MediMax

How to Prevent Disease

Of course, it is always better to try to keep your fish as healthy as possible at all times to reduce the chance of a disease outbreak. So how do you prevent koi disease? Most importantly – keep your pond environment pristine (poor water quality stresses fish and reduces their immune systems effectiveness). By maintaining optimal water quality, your fish will be healthy, their immune systems will be strong and the fish will be more able to fight off any potential disease threats.

  • Maintain optimal water quality in your pond, poor water quality is probably the main cause of disease outbreaks. You can achieve this by not overcrowding your fish, installing an excellent filtration system and performing proper pond maintenance.
  • Quarantine all new arrivals. New fish, no matter how well the have been quarantined at the koi dealer may bring parasites into your pond. Read our Koi Quarantine Procedure article before adding new fish  to your pond, this will greatly reduce the risk of introducing any new disease to your existing fish.
  • Perform regular filter maintenance, water changes and water quality testing.


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