What does Koi herpesvirus (KHV) (Cyprinid herpesvirus 3) look like?

Color fade. Refusing food. Lethargy.

How is Koi herpesvirus (KHV) (Cyprinid herpesvirus 3) treated?

No cure exists: KHV has no antiviral treatment. Fish that survive carry the virus for life and shed it intermittently.

POND disease

Koi herpesvirus (KHV) (Cyprinid herpesvirus 3)

Reviewed by the Fast Aquatics husbandry team · Updated May 2026

Quick referenceKoi herpesvirus (KHV) is a lethal viral infection of koi + common carp. No cure. 80-100% mortality once symptoms appear; isolation + biosecurity are the only defenses.

Symptoms

Color fade
Refusing food
Lethargy
Surface gasping
Mucus on gills
Bulging eye(s)
Sunken eyes
Notched fins

Cause

Cyprinid herpesvirus 3 (CyHV-3). Highly contagious between koi + common carp. Transmits via water, infected fish, contaminated nets/tools/hands. Triggered by water temperatures of 64-77°F (18-25°C).

Treatment options

No cure exists. KHV has no antiviral treatment. Fish that survive carry the virus for life and shed it intermittently.
Heat shock (controversial). Some breeders attempt 86°F heat for 30 days. Limited success. Pathogen-shedding survivors are still carriers.
Cull + sterilize. Most pond keepers euthanize the entire stock + sterilize the system. Drain pond, bleach 10% all surfaces, dry 4-6 weeks before restocking.
Quarantine all survivors. If keeping survivors: permanent quarantine. Never mix with KHV-naive fish.

Always treat in a separate quarantine or hospital tank. Most medications are toxic to coral, invertebrates, and live rock biology. Consult an aquatic veterinarian for valuable fish.

Prevention

Quarantine new koi 60+ days at 75°F to express virus. Never share nets/tools between ponds. Avoid wild-caught carp + Israeli/Asian-imported koi without health certificates. Buy from KHV-tested breeders only.

Fatality + outcome

Catastrophic - 80-100% mortality once symptoms appear. Tank-killer.

Full disease database · Symptom matcher · Q&A library · Glossary · QT timeline calculator

Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.