KHV — Koi Herpesvirus, the Most Feared Disease
Koi Herpesvirus (KHV) is the most serious viral disease in the koi hobby, with mortality rates reaching 80–100% in affected ponds. KHV is a notifiable disease in many countries and has caused catastrophic losses in commercial and hobbyist collections worldwide. The virus attacks gill tissue, causing progressive gill necrosis (tissue death) that prevents oxygen uptake — affected koi essentially suffocate despite being in water. There is no effective treatment for KHV; management is entirely focused on prevention and quarantine.
KHV clinical signs include gasping at the surface, gill pallor (pale or mottled gills visible on close inspection), sunken eyes, skin lesions, and rapid mass mortality. The disease is temperature-dependent — it is most active and lethal at 18–25°C. In Cambodia, where water temperatures often exceed 28°C, KHV activity is reduced but not eliminated. Diagnosis requires laboratory PCR testing; visual diagnosis is unreliable. If you suspect KHV, immediately isolate surviving fish and contact an aquatic veterinarian.
Prevention through strict quarantine is the only reliable protection against KHV. Every new koi introduced to an established pond represents a potential KHV vector — even apparently healthy fish can be asymptomatic carriers. The minimum quarantine period is 3–4 weeks in a separate tank, ideally at a temperature that activates latent KHV if present (22–25°C). In Cambodia, where koi often change hands through markets with unknown history, the risk of purchasing KHV-positive fish is real and quarantine is non-negotiable for serious collectors.
- ✦Never add new koi directly to your main pond — minimum 3–4 weeks quarantine in a separate tank without exception
- ✦If you attend a koi show or exhibition, disinfect any nets or equipment before returning them to your pond
- ✦Source koi from suppliers with KHV-tested stock — ask for documentation and be willing to pay more for verified clean fish
Ich (White Spot) — Common but Treatable
Ich (Ichthyophthirius multifiliis) is the most common disease in all freshwater fish including koi, identifiable by small white spots (1–2mm, resembling sprinkled salt) on the skin and fins. The parasite has a complex life cycle: the feeding stage (trophont) burrows under the fish's skin and mucous membrane, causing irritation; it then drops off as a reproducing cyst (tomont) that releases hundreds of infective free-swimming theronts that seek new hosts. Treatment must target the free-swimming theront stage — the parasite is protected under the skin while in its feeding stage.
Koi infected with Ich display characteristic behaviors: flashing (rubbing against pond surfaces), rapid gill movement, clamped fins, and reduced appetite — often before the white spots are visible. In Cambodia's warm water (26–32°C), the Ich life cycle completes very rapidly — as fast as 3–4 days at 30°C compared to 3–5 weeks at 10°C. This means outbreaks can escalate from a few spots to full-body coverage in 72 hours if not treated immediately.
Effective Ich treatments include malachite green, formalin-malachite combinations, and more recently potassium permanganate (KMnO4) which is available in Cambodia. Salt (sodium chloride) at 3g per liter provides some treatment effect but is insufficient as the sole treatment for severe Ich infestations. Raise water temperature to 28–30°C (to accelerate the parasite life cycle and expose the vulnerable theront stage) and treat for a full 7-day course. Remove activated carbon from filters during treatment as it absorbs medications.
- ✦Remove activated carbon from all filters before any chemical treatment — carbon immediately adsorbs and neutralizes medications
- ✦Treat for the full recommended course even after white spots disappear — the parasite may still be in the encysted reproductive stage
- ✦After successful Ich treatment, do a 30% water change before reintroducing carbon filtration
Anchor Worm and Fish Lice — External Parasites
Anchor worm (Lernaea) is a parasitic crustacean that buries its anchor-shaped head into the fish's muscle tissue and hangs visibly from the body as a 1–2 cm white or greenish thread-like appendage. Despite its name it is not actually a worm. Anchor worm causes significant physical damage, inflammation at the attachment site, and secondary bacterial infections. In Cambodia, anchor worm is commonly introduced through fish from markets or ponds that use outdoor water sources shared with wild fish.
Fish lice (Argulus) are flat, disc-shaped crustacean parasites up to 10mm across, visible to the naked eye on the skin as moving oval shapes. They use sharp mouth parts to puncture the skin and feed on blood and mucus, leaving circular wounds that invite bacterial and fungal secondary infections. Fish lice are highly mobile and can transfer between fish in the same pond rapidly. Infected koi show intense flashing behavior, become lethargic, and may refuse food.
Treatment for both anchor worm and fish lice uses organophosphate treatments (Masoten/trichlorfon) at carefully measured doses, or manual removal followed by topical treatment of wound sites. Manual removal requires gently gripping the parasite with forceps as close to the attachment point as possible and pulling smoothly — breaking the anchor worm at the surface leaves the buried head and greatly worsens the wound. After manual removal, treat the wound site with potassium permanganate paste or iodine and place the fish in a salt bath (3–5g/L) for 30 minutes before returning to a clean, treated pond.
- ✦Inspect all new fish thoroughly in bright light before quarantine — anchor worm is visible to the naked eye on careful inspection
- ✦Treat the entire pond when fish lice are found on one fish — Argulus is highly mobile and will already be on multiple fish
- ✦Potassium permanganate (KMnO4) treats anchor worm, fish lice, bacterial infections, and fungal disease simultaneously — a versatile Cambodia-available treatment
Bacterial Infections — Fin Rot and Ulcer Disease
Fin rot (Aeromonas or Pseudomonas bacteria) is one of the most common bacterial infections in koi, presenting as fraying, discoloration, and progressive erosion of fin edges. Early fin rot shows as white edges on fin tips; advanced cases erode the fin tissue to the base, potentially exposing bone. The bacteria are ubiquitous in pond water and typically infect only fish that are stressed, injured, or immunocompromised by poor water quality. Treating fin rot without addressing the underlying cause (usually poor water quality or overcrowding) always results in recurrence.
Ulcer disease (hemorrhagic septicemia caused by motile Aeromonas) is more serious — deep, crater-like wounds that penetrate through the scales and skin into the muscle tissue. Ulcers typically appear as round red sores 1–5 cm in diameter, often with raised white edges and a sunken center. Untreated ulcers can expose bone and organs within weeks, and the bacteria can spread to internal organs causing lethal septicemia. Ulcers in koi require prompt, aggressive treatment: topical application of antiseptic (iodine, hydrogen peroxide) under anesthesia, injection antibiotics in severe cases, and systemic antibiotic food.
Antibiotic treatment for bacterial koi diseases in Cambodia requires either medicated food (antibiotics added to pellets) or, for severe cases, injection under veterinary guidance. Enrofloxacin and amoxicillin are commonly used. Importantly, antibiotic treatment temporarily disrupts the pond's beneficial bacteria colony — combine treatment with beneficial bacteria supplements (available at pet shops) and increase water changes to manage the temporary increase in ammonia while the filter bacteria recover.
- ✦For minor fin rot, salt at 3g/L and improved water quality is often sufficient — reserve antibiotics for progressing infections
- ✦Treat ulcers as soon as spotted — ulcers that are visible on the outside are already deep; delay is dangerous
- ✦Always complete the full antibiotic course — stopping early creates antibiotic-resistant bacteria and allows infection to return
Salt Baths, Quarantine, and Finding Help in Cambodia
Salt (sodium chloride — pond salt, not table salt with iodine) is the most versatile and accessible treatment tool for koi disease in Cambodia. Salt baths at 3–5g per liter for 30-minute treatment sessions address many external parasites, reduce osmotic stress in sick fish, and inhibit several bacterial and fungal pathogens. A long-term pond salt dose of 1–2g per liter provides ongoing immune support and reduces the impact of nitrite toxicity. Salt is inexpensive, widely available, and safe when used correctly — it is the first tool every Cambodian koi keeper should have.
A quarantine tank setup is arguably as important as the main pond. A 200–500 liter plastic tank or fiberglass container with a reliable sponge filter, heater (to maintain stable temperature during treatment), and aeration forms the treatment hospital that every koi keeper needs. Fish can be isolated and observed, treated with medications without affecting the main pond's bacterial colony, and monitored closely during recovery. Never attempt to treat a serious disease in the main pond if you can avoid it — the disruption to the pond ecosystem compounds the stress on the patient fish.
Finding aquatic veterinary expertise in Cambodia is challenging but improving. As of 2026, several veterinary clinics in Phnom Penh have staff with basic fish medicine knowledge, particularly those serving agricultural and commercial aquaculture clients. For serious conditions (KHV diagnosis, injection antibiotics, surgical ulcer treatment), contact aquaculture departments at the Royal University of Agriculture in Phnom Penh or established koi hobbyist groups on social media who can recommend experienced practitioners. International online consultation with fish veterinarians via video call is an increasingly practical option for diagnosis when local expertise is limited.
- ✦Use only non-iodized pond salt or kosher salt for koi treatment — iodized table salt burns koi skin and gills
- ✦Maintain a basic fish medicine kit: salt, potassium permanganate, formalin, aquarium antibiotic food, methylene blue (for fungal treatment)
- ✦Document symptoms with photos and video before treatment — helpful when consulting vets or online experts remotely from Cambodia