Knowledge Base
Koi Health Guide
A reference guide to common Koi diseases, their symptoms, and evidence-based treatment protocols. Filter by symptom to identify potential issues with your fish.
Showing 10 of 10 conditions
Koi Herpesvirus (KHV)
A highly contagious and often fatal herpesvirus affecting common and koi carp. It spreads rapidly at water temperatures of 18–28°C and causes severe gill and organ damage.
Common Symptoms
- Lethargy
- Gill necrosis / pale gills
- Sunken eyes
- Skin lesions & ulcers
- Gasping at surface
- Rapid mass mortality
Bacterial Ulcer Disease
Caused by opportunistic bacteria such as Aeromonas and Pseudomonas, typically triggered by stress, poor water quality, or injury. Wounds can rapidly deepen if untreated.
Common Symptoms
- Open ulcers on body
- Red or inflamed skin
- Fin rot / fraying
- Lethargy
- Loss of appetite
White Spot (Ich / Ichthyophthirius)
One of the most common Koi diseases, caused by the ciliate parasite Ichthyophthirius multifiliis. It burrows into skin and gills, causing intense irritation. The free-swimming stage is the only treatable phase.
Common Symptoms
- Small white spots (salt-like) on body & fins
- Flashing / rubbing against surfaces
- Clamped fins
- Lethargy
- Increased mucus production
Anchor Worm (Lernaea)
A macroparasitic crustacean that burrows its anchor-shaped head into Koi tissue. Females are visible as thread-like worms on the body surface. Secondary bacterial infections are common at attachment sites.
Common Symptoms
- Visible worm-like parasites attached to skin
- Redness & inflammation at attachment site
- Ulcers after removal
- Flashing / rubbing
- Restlessness
Fish Lice (Argulus)
Argulus is a branchiuran crustacean (fish louse) that attaches to the skin and injects digestive enzymes to feed on blood. It causes significant irritation and can transmit other pathogens.
Common Symptoms
- Disc-shaped parasites visible on body
- Intense flashing & rubbing
- Red spots / irritation marks
- Lethargy
- Appetite loss
Flukes (Gill & Skin)
Monogenean flatworm parasites — Dactylogyrus (gill flukes) and Gyrodactylus (skin flukes). Gill flukes are egg-layers; skin flukes are live-bearers. Both cause significant irritation and secondary infections.
Common Symptoms
- Gasping at surface
- Flashing / scratching
- Clamped fins
- Excess mucus on gills or skin
- Pale or swollen gills
- Lethargy
Dropsy (Pinecone Disease)
Dropsy is a syndrome rather than a single disease — it is caused by fluid accumulation in body cavities, often the result of systemic bacterial infection (Aeromonas), organ failure, or viral disease. Prognosis is often poor.
Common Symptoms
- Swollen / bloated abdomen
- Raised scales (pinecone appearance)
- Protruding eyes (Popeye)
- Lethargy
- Loss of appetite
- Dark colouration
Swim Bladder Disorder
Swim bladder disorder affects a Koi's ability to regulate buoyancy. It can be caused by bacterial infection, constipation, overfeeding, rapid temperature changes, or physical trauma.
Common Symptoms
- Floating at surface
- Sinking to bottom
- Tilting or swimming sideways
- Inability to maintain position
- Sluggish movement
Columnaris (Saddleback Disease)
Caused by Flavobacterium columnare, a gram-negative bacterium that thrives in warm, low-oxygen water. It is highly contagious and can kill fish within 24–48 hours at high temperatures.
Common Symptoms
- Saddle-shaped grey/white lesion behind dorsal fin
- Frayed or rotting fins
- Ulcers on body
- Lethargy
- Rapid deterioration
Fungal Infection (Saprolegnia)
Saprolegnia is a water mould that attacks damaged or stressed tissue. It is almost always secondary to another injury or illness. Spores are present in all pond water but only cause infection when immunity is compromised.
Common Symptoms
- White or grey cotton-wool patches on body/fins
- Fuzzy tufts around wounds or eyes
- Redness under fungal growth
- Lethargy
- Appetite loss
This guide is for informational purposes only. Always consult a qualified aquatic veterinarian for a definitive diagnosis and treatment plan.