Koi Disease Identification: Signs, Diagnosis, and Treatment Pathways

By KoiQuanta Editorial Team|Updated Mar 2026

Getting the diagnosis wrong is one of the most expensive mistakes in koi keeping. You treat for parasites when the fish has a bacterial infection. You use antibiotics on a viral disease that can't be treated. You miss the early signs and a manageable problem becomes a mortality event.

Here's a practical guide to identifying what's wrong with your fish.

TL;DR

(Be precise - "looks sick" isn't useful) 2.
multiple changes the diagnosis significantly) 3.
KoiQuanta connects observations, water data, and treatment records in one searchable history.
Early detection based on parameter trends reduces treatment costs and fish stress.
Seasonal changes require adjusted monitoring schedules; automated reminders help maintain consistency.

How to Approach a Sick Koi

Before you reach for any medication, answer these questions:

What are the specific signs? (Be precise - "looks sick" isn't useful)
Which fish are affected? (One fish vs. multiple changes the diagnosis significantly)
When did signs first appear?
What's the water temperature?
What are the water parameters? (Test before doing anything else)
Were any new fish added recently?
Has anything changed in the system?

Water quality problems cause disease signs. A fish gasping at the surface may have gill flukes, KHV, or may be suffocating from ammonia toxicity. Test the water first. You'd be surprised how often "sick fish" resolves with a water change.

Disease Identification by Primary Sign

Rapid Gilling / Gasping at Surface

First check: Ammonia and nitrite levels, dissolved oxygen. Oxygen depletion and ammonia toxicity both cause gasping.

If water is fine:

Gill flukes (Dactylogyrus) - most common cause, especially in new fish
Gill bacterial disease - pale, swollen gills visible at cover edge
KHV - especially if multiple fish affected simultaneously, at 59-77°F
Velvet - check for gold dust on skin under bright light

Action: Test water. If clean, treat for gill flukes with praziquantel. Escalate to KHV testing if multiple fish crash.

White Spots

Ich (Ichthyophthirius multifiliis): Distinct white spots, grain-of-salt size, distributed on fins and body. Fish flash and scratch.

Lymphocystis: White to gray wart-like growths, usually on fins. Not contagious - viral, resolves on its own. Don't confuse with ich.

Anchor worm egg cases: Small white protrusions, usually in fin tissue or body.

Excess mucus clumping: White patches that look irregular, not distinct spots.

Action: Ich requires treatment (salt + heat or malachite green/formalin). Lymphocystis requires no treatment. Confirm visually before treating.

Skin Lesions / Ulcers

Aeromonas ulcer disease: Circular or irregular tissue loss, often crater-like. Red margins. Can be single or multiple sites.

Physical trauma: Often a single irregular wound, not circular. Check for evidence of predator attack, fin damage from sharp edges.

Spawning wounds: Ragged fin damage, scale loss along flanks in spring - spawning behavior causes significant abrasion.

Carp pox: Gray or white waxy smooth raised areas, usually on fins. Viral, resolves with warm temperatures, no treatment.

Action: Bacterial ulcers require isolation, wound care, and antibiotics. Physical trauma requires wound care and antibiotics for secondary infection prevention. Carp pox requires nothing.

Fin Rot

Bacterial fin erosion: Progressive erosion from the fin margin inward. Ragged edge, often reddish at the base. Caused by Aeromonas, Pseudomonas, or Flavobacterium.

Fluke damage: Can cause fin margin erosion from attachment and feeding damage. Typically less organized-looking than bacterial fin rot.

Action: Isolate. Improve koi pond water quality tracker. Antibiotic treatment. If flukes are present, treat those first - fin rot often secondary to fluke damage.

Swollen Abdomen / Raised Scales (Pineconing)

Dropsy (pinecone appearance - scales standing out from body): Usually a late-stage bacterial infection causing fluid accumulation. Aeromonas most common cause. Prognosis is poor once pineconing is established.

Pregnancy/Egg retention: Female koi can appear very swollen with eggs in spring. Scales don't raise - the body shape is uniformly rounded, not pineconed.

Coelomic fluid accumulation (various causes): Swelling without pineconing. Can indicate kidney disease, polycystic kidneys, or other organ issues.

Action: Dropsy - isolate, antibiotic treatment, Epsom salt baths. Prognosis poor if pineconing is pronounced. Don't confuse with gravid females in spring.

Flashing / Scratching Behavior

Flukes: Most common cause. Gyrodactylus especially common in newly arrived fish.

Ich: Usually accompanied by white spots, but flashing can precede visible spots.

Trichodina: Unicellular parasite. Causes flashing. Common in poor water quality.

Costia (Ichthyobodo): Causes flashing and excess mucus. Microscopy needed for definitive ID.

pH or chemical irritation: Water chemistry problems can cause flashing. Test parameters.

Action: If new fish: prophylactic praziquantel for flukes, salt to 0.3%. If established fish start flashing: test water, consider skin scrape, treat based on findings.

Sunken Eyes / Exophthalmia (Pop-Eye)

Pop-eye (exophthalmia): One or both eyes protruding abnormally. Usually bacterial infection. Can be associated with systemic Aeromonas infection.

Sunken eyes (enophthalmia): Eyes appearing to sink inward - dehydration (very late-stage disease), or KHV sign.

Action: Pop-eye - isolate, antibiotic treatment. If KHV is suspected (multiple fish declining, sunken eyes, gill signs), get PCR testing.

Disease-Specific Identification

KHV (Koi Herpesvirus)

Signs: Mass respiratory distress in multiple fish, gill necrosis, sunken eyes, skin blotching, rapid mortality. Temperature: 59-77°F.

Diagnosis: PCR testing.

Treatment: None. Reportable disease. Contact state veterinarian.

Spring Viremia of Carp (SVC)

Signs: Hemorrhages on fins, skin, and eyes. Swollen abdomen. Lethargy. Temperature: 41-63°F.

Diagnosis: Lab testing required.

Treatment: None. Reportable disease.

Aeromonas / Bacterial Ulcers

Signs: Ulcers, fin rot, hemorrhagic septicemia, dropsy.

Diagnosis: Clinical signs, culture and sensitivity testing for antibiotic selection.

Treatment: Antibiotics, wound care. Isolate.

Ich

Signs: White spots, flashing, elevated gilling.

Diagnosis: Visual - spots visible to naked eye.

Treatment: Salt + heat, or malachite green/formalin products.

Monogenean Flukes

Signs: Flashing, elevated mucus, respiratory distress.

Diagnosis: Skin or gill scrape.

Treatment: Praziquantel (two doses).

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Sources

Associated Koi Clubs of America (AKCA)
Koi Organisation International (KOI)
University of Florida IFAS Extension Aquaculture Program
Fish Vet Group
Water Quality Association