How do I identify what disease my koi has?

Start with four inputs: visual symptoms (spots, lesions, swelling, fin damage), behavioural signs (flashing, surface hovering, isolation), current water quality parameters, and recent history (new fish, temperature change, treatment). The combination of these narrows your differential diagnosis much more effectively than visual appearance alone. KoiQuanta's symptom checker uses all four inputs to give you a probability-ranked diagnosis with direct links to the relevant treatment protocol.

What are the most common koi diseases?

The most frequently encountered are Aeromonas ulcer disease, Ich (white spot), gill and skin flukes, Trichodina and Costia, anchor worm and fish lice, Saprolegnia water mould, and Columnaris. Most of these are opportunistic: they exploit stress, poor water quality, or immune suppression. Good water quality management, regular testing, and structured quarantine for new fish prevent the majority of common koi disease introductions.

What does a healthy koi look like?

A healthy koi swims actively with an erect dorsal fin, feeds eagerly, has clear and bright eyes, clean smooth skin without lesions or excess mucus, intact fins, and no abdominal swelling. Any deviation, particularly sudden changes in appetite, activity, or appearance, warrants immediate investigation with a water test and close physical examination. ---

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Koi Disease Identification Guide: Symptoms, Causes, and Treatments

By KoiQuanta Editorial Team|Updated Mar 2026

Early disease identification and treatment reduces koi mortality rates by over 70% compared to delayed or misdiagnosed treatment. The window between first symptoms and a crisis is often measured in days, sometimes hours.

This koi disease identification guide walks you through the most common koi diseases by symptom: what to look for, what causes it, and what to do next. For each condition, KoiQuanta's treatment tracker connects your diagnosis to the right protocol.

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