Before You Begin: The Golden Rules of Koi Health
1. Quarantine every new fish for a minimum of 4–6 weeks.
Never introduce a new koi directly into your main pond. A dedicated quarantine tank (minimum 100 gallons, with filtration and aeration) is non-negotiable. Most diseases are introduced through new fish.
2. Test your water before you treat your fish.
Poor water quality is the #1 cause of koi illness. Before assuming disease, test for: ammonia (0 ppm), nitrite (0 ppm), nitrate (<40 ppm), pH (7.0–8.5), dissolved oxygen (>6 mg/L), and temperature. Many "diseases" resolve with a water change.
3. Isolate sick fish immediately.
At the first sign of illness, move the affected fish to a quarantine/hospital tank. This protects your pond population and allows you to treat with medications that would harm your biological filter.
4. Never mix medications without research.
Some medications are lethal in combination. Treat one condition at a time unless you have confirmed co-infections.
Quarantine Tank Setup
Every koi keeper should have a hospital tank ready before they need it.
- Size: Minimum 100 gallons (larger is better)
- Filtration: Sponge filter or established biological filter (avoid carbon — it removes medications)
- Aeration: Strong air stone — many medications reduce dissolved oxygen
- Heater: Adjustable — some treatments require elevated temperatures
- Cover: Koi jump when stressed
- Salt: Keep non-iodized pond salt on hand at all times (Morton's Pool Salt or Cargill Solar Salt)
- Lighting: Subdued — sick fish are stressed by bright light
- No substrate: Bare bottom for easy cleaning and monitoring
Disease 1: Ich (White Spot Disease) — Ichthyophthirius multifiliis
What It Is
Ich is a parasitic protozoan that burrows under the skin and fins of koi. It is one of the most common and recognizable koi diseases.
Signs to Look For
- White spots resembling grains of salt or sugar on the body, fins, and gills
- Fish rubbing or "flashing" against pond walls, rocks, or the bottom
- Clamped fins
- Lethargy and loss of appetite
- Rapid gill movement (if gills are affected)
- In severe cases, fish may gasp at the surface
Diagnosis
Visual identification is usually sufficient. Spots are 0.5–1mm white cysts. Confirm with a skin scrape under a microscope if uncertain — you'll see the characteristic rotating parasite inside the cyst.
Treatment Protocol
Step 1 — Quarantine: Move affected fish to hospital tank immediately.
Step 2 — Raise Temperature: Gradually raise water temperature to 80–82°F (27–28°C). The ich life cycle accelerates at higher temperatures, making the parasite vulnerable faster. Do not exceed 86°F.
Step 3 — Salt Treatment: Add non-iodized pond salt at 0.3% concentration (3 lbs per 100 gallons). Salt disrupts the osmotic balance of the parasite.
Step 4 — Medication:
- Ich-X (Hikari) — formalin and malachite green based; highly effective. Dose per label instructions, typically every 24 hours for 3 treatments.
- Kordon Rid-Ich Plus — formalin and malachite green; widely available.
- Malachite Green (standalone) — effective but handle with gloves; carcinogenic.
- Potassium Permanganate (KMnO₄) — 2–4 ppm bath treatment; effective for heavy infestations.
Suppliers: Hikari USA (hikariusa.com), Kordon (kordonvet.com), Pond Megastore (pondmegastore.com)
Step 5 — Continue Treatment: Treat for a minimum of 10–14 days after the last visible spot disappears. The encysted stage is immune to medication — you must catch the free-swimming stage.
Disease 2: Anchor Worm — Lernaea spp.
What It Is
Anchor worm is a parasitic crustacean that embeds its anchor-shaped head into the muscle tissue of koi, causing significant irritation and secondary infections.
Signs to Look For
- Visible worm-like protrusions (1–2 cm) attached to the body, fins, or around the eyes
- Red, inflamed attachment sites
- Fish rubbing against surfaces
- Ulcers or open wounds at attachment points
- Lethargy
Treatment Protocol
Step 1 — Manual Removal: Using sterilized tweezers, grasp the anchor worm as close to the skin as possible and pull straight out with a slow, steady motion. Do not twist — the anchor head must come out intact or the wound will become infected.
Step 2 — Wound Treatment: Apply Tricide-Neo (Molecular Therapeutics), Betadine, or Melafix (API) directly to the wound.
Step 3 — Chemical Treatment: Dimilin (diflubenzuron) at 0.066 ppm is the gold standard. Treat every 7 days for 3–4 treatments. Also effective: Potassium Permanganate (2–4 ppm bath) or Lice & Anchor Worm (API).
Suppliers: Molecular Therapeutics (fishvet.com), API (apifishcare.com), Pond Megastore, Koi Acres, Amazon
Disease 3: Fish Lice — Argulus spp.
What It Is
Fish lice are disc-shaped parasitic crustaceans that attach to the skin of koi and feed on blood and tissue fluids.
Signs to Look For
- Visible disc-shaped parasites (3–8mm) on the body — flat, oval, semi-transparent
- Intense flashing and rubbing
- Red, irritated patches at attachment sites
- Restlessness and erratic swimming
- Secondary bacterial infections at bite sites
Treatment Protocol
Dimilin (diflubenzuron) at 0.066 ppm is the most effective treatment. Repeat every 7 days for 3–4 cycles. Also effective: Potassium Permanganate (2–4 ppm), Lice & Anchor Worm (API), or Cyromazine. Treat bite wounds with Tricide-Neo or Betadine to prevent secondary infection.
Disease 4: Flukes — Gyrodactylus (Skin) & Dactylogyrus (Gill)
What It Is
Flukes are microscopic monogenean flatworms that attach to the skin or gills of koi. They are extremely common and often present in low numbers without causing disease — problems arise when fish are stressed or water quality declines.
Signs to Look For
Skin Flukes: Flashing, excess mucus, reddened skin, clamped fins, lethargy.
Gill Flukes: Rapid or labored breathing, gasping at the surface, one or both gill covers held open, pale or inflamed gills.
Treatment Protocol
Praziquantel is the gold standard for flukes. Dose at 2–4 mg/L for 5–7 days. Available as PraziPro (Hikari), Prazi-Pond Wormer (Koi Care Kennel), or General Cure (API). Repeat treatment at day 5–7 and again at day 10–14 to catch newly hatched flukes — praziquantel does not kill eggs.
Suppliers: Hikari USA (hikariusa.com), Koi Care Kennel (koicarekennel.com), API (apifishcare.com)
Disease 5: Bacterial Ulcer Disease (Aeromonas / Pseudomonas)
What It Is
Bacterial ulcers are open wounds caused by opportunistic bacteria — most commonly Aeromonas hydrophila and Pseudomonas fluorescens. They are the most common serious disease in koi and are almost always secondary to stress, injury, or parasites.
Signs to Look For
- Open sores or ulcers on the body — may start as a red spot and progress to a deep, crater-like wound
- Red streaking around the wound (hemorrhagic septicemia)
- Raised scales around the wound
- Fin rot (fraying or disintegration of fins)
- Lethargy and loss of appetite
- Pinecone appearance (scales standing out) — indicates systemic infection
Treatment Protocol
Step 1 — Quarantine.
Step 2 — Wound Cleaning: Remove fish from water, wrap in wet towel, clean wound with sterile gauze, apply Betadine, seal with Orabase or Neosporin. For deep wounds, pack with Tricide-Neo soaked gauze.
Step 3 — Antibiotic Treatment:
- Tricide-Neo dip — 5-minute dip daily for 7–10 days; highly effective for surface ulcers
- Medicated food — Romet-B, Medikoi Antibiotic (NT Labs), or Koi Care Kennel Antibiotic Food
- Injectable antibiotics (veterinarian) — Enrofloxacin (Baytril) or Oxytetracycline for severe systemic infections
Step 4: Maintain 0.3% salt and perform daily 25–30% water changes in the hospital tank.
Suppliers: Molecular Therapeutics (fishvet.com), Koi Care Kennel (koicarekennel.com), NT Labs (ntlabs.co.uk)
Disease 6: Dropsy (Pinecone Disease)
What It Is
Dropsy is a symptom of systemic organ failure — most commonly kidney failure — causing fluid accumulation in the body cavity. It is often caused by bacterial infection (Aeromonas) but can also result from viral infection, parasites, or poor water quality.
Signs to Look For
- Scales standing out from the body in a "pinecone" pattern — the most distinctive sign
- Severely bloated abdomen
- Protruding eyes (exophthalmia/pop-eye)
- Pale or reddened gills
- Lethargy and loss of appetite
Treatment Protocol
Quarantine immediately. Add Epsom salt at 1 tsp per 10 gallons. Antibiotic treatment: Kanaplex (Seachem) or Furan-2 (API). If fish is still eating, antibiotic medicated food is the most effective delivery method. Maintain pristine water quality with daily water changes. Prognosis is poor in advanced cases.
Disease 7: Koi Herpesvirus (KHV)
What It Is
Koi Herpesvirus is a highly contagious and often fatal viral disease. It is a notifiable disease in many countries. There is no cure.
Signs to Look For
- Rapid onset of mass mortality
- Sunken eyes
- Pale or mottled, necrotic gills
- Skin lesions and blisters
- Gasping at the surface
- Temperature-dependent: most active between 60–77°F (16–25°C)
Management
KHV can only be definitively diagnosed through PCR testing. Do not move fish if KHV is suspected. Euthanize affected fish humanely, disinfect all equipment with 10% bleach, and report to USDA APHIS (aphis.usda.gov). Prevention: quarantine all new fish for 6 weeks and purchase only from reputable sources.
Disease 8: Spring Viremia of Carp (SVC)
What It Is
Spring Viremia of Carp is a viral hemorrhagic disease most active in spring at water temperatures of 46–64°F (8–18°C). It is a reportable disease with no cure.
Signs to Look For
- Hemorrhaging from the skin, gills, eyes, and anus
- Bloated abdomen and protruding eyes
- Darkened skin and lethargy
- Mass mortality
Management
PCR testing only. Euthanize affected fish, disinfect all equipment, and report to your state veterinarian or USDA APHIS.
Disease 9: Columnaris (Saddleback Disease) — Flavobacterium columnare
What It Is
Columnaris is a bacterial infection affecting the skin, fins, and gills. It progresses very rapidly — treat immediately upon suspicion.
Signs to Look For
- White or gray lesions on the back, often with a "saddle" shape across the dorsal area
- Fraying fins with white edges
- Gill necrosis — gills appear pale and deteriorating
- Rapid breathing and lethargy
Treatment Protocol
Quarantine. Maintain 0.3% salt. Medication: Furan-2 (API) as first-line treatment, Kanaplex (Seachem) for resistant strains, or Tetracycline as medicated food. Tricide-Neo dip for surface lesions. Begin treatment within hours of diagnosis.
Disease 10: Fungal Infections — Saprolegnia spp.
What It Is
Fungal infections in koi are almost always secondary — they colonize damaged tissue caused by injury, parasites, or bacterial infection.
Signs to Look For
- White, gray, or brown cotton-like tufts on the skin, fins, or around the eyes
- Lesions that appear fuzzy or fluffy
- Usually associated with an underlying wound or ulcer
Treatment Protocol
Address the underlying cause first. Maintain 0.3% salt. Medication options: Methylene Blue (mild cases), Pimafix (API), Potassium Permanganate (2–4 ppm bath), Formalin (25 ppm bath for 1 hour with strong aeration), or Malachite Green for severe cases.
Disease 11: Pop-Eye (Exophthalmia)
What It Is
Pop-eye is a symptom where one or both eyes protrude abnormally due to fluid accumulation behind the eye, most commonly caused by bacterial infection.
Signs to Look For
- One or both eyes visibly bulging from the head
- Cloudiness of the eye and redness around the eye socket
Treatment Protocol
Kanaplex (Seachem) is the most effective antibiotic for pop-eye. Furan-2 (API) as an alternative. Add Epsom salt at 1 tsp per 10 gallons. Unilateral pop-eye (one eye) caused by injury may resolve with good water quality alone. Bilateral pop-eye requires aggressive antibiotic treatment.
Disease 12: Swim Bladder Disorder
What It Is
The swim bladder controls buoyancy. Disorders can be caused by bacterial infection, physical injury, constipation, or genetic defects.
Signs to Look For
- Fish swimming upside down or sideways
- Fish unable to maintain normal depth — sinking or floating
- Bloated abdomen or curved spine
Treatment Protocol
Fast the fish for 3–7 days. Feed shelled, cooked peas (natural laxative). Add Epsom salt at 1 tsp per 10 gallons. If bacterial infection is suspected, treat with Kanaplex or Furan-2. Severe cases may require veterinary intervention.
Quick Reference: Medication Suppliers
- Hikari USA — PraziPro, Ich-X, medicated foods — hikariusa.com
- Seachem — Kanaplex, Metroplex, ParaGuard — seachem.com
- API — Furan-2, General Cure, Melafix, Pimafix — apifishcare.com
- Molecular Therapeutics — Tricide-Neo — fishvet.com
- Koi Care Kennel — Prazi, antibiotic food, full koi health line — koicarekennel.com
- NT Labs — Medikoi Antibiotic, Fluke-M — ntlabs.co.uk
- Pond Megastore — Full range of pond treatments — pondmegastore.com
- Kordon — Rid-Ich Plus, Methylene Blue — kordonvet.com
When to Call a Veterinarian
- Suspected KHV or SVC (reportable diseases)
- Fish not responding to treatment after 7–10 days
- Need for prescription antibiotics (Baytril, Romet-B)
- Surgical intervention (swim bladder, tumor removal)
- Mass mortality events
Find an aquatic veterinarian through the World Aquatic Veterinary Medical Association (WAVMA) at wavma.org or the American Association of Fish Veterinarians at aafv.org.
Final Thoughts
The best treatment is prevention. Maintain excellent water quality, quarantine every new fish, feed a high-quality diet, and observe your koi daily. Most diseases are caught early by keepers who know their fish well enough to notice when something is off — a change in swimming behavior, a fish that isn't eating, or a subtle color change can be the first warning sign.
At Crystal Creek Koi Farm, we quarantine every fish before it leaves our facility and feed our koi Blackwater premium koi food to support immune health and vibrant color. Healthy fish start with healthy practices.