Almost every koi disease starts with a water quality problem or a fish added without quarantine. Before you treat anything, test ammonia, nitrite, pH and temperature — a large proportion of the symptoms below resolve on water changes alone, and medicating a pond whose real problem is ammonia makes things worse.
Symptom Index
| What you see | Most likely causes | First action |
|---|---|---|
| Flashing — rubbing or scraping against the pond floor | Skin or gill parasites; high ammonia, nitrite or chlorine | Test water; then scrape/scope for parasites |
| White grains over body and fins | Ich (white spot) | Salt at 0.3% plus temperature management |
| Lying on the bottom, barely moving | Cold-water torpor (normal); low oxygen; ammonia; parasites; KHV | Check temperature and oxygen first |
| Gasping at the surface, crowding the waterfall | Low dissolved oxygen; gill damage | Add aeration immediately, water change |
| Not eating | Water below 50°F (normal); water quality; stress; illness | Check thermometer, then test water |
| Ragged, receding fins with white or red edges | Fin rot (bacterial, secondary to poor water) | Water changes; salt; antibacterial if progressing |
| Red sores or open ulcers on the flank | Aeromonas / Pseudomonas ulcer disease | Isolate, topical treatment, vet consult |
| Body swollen, scales sticking out like a pinecone | Dropsy — internal organ failure, often terminal | Isolate; prognosis poor |
| Cotton-wool tufts on skin | Fungus (Saprolegnia), always secondary to injury | Treat the underlying wound and water quality |
| Thread-like growths with two barbs at the attachment | Anchor worm (Lernaea) | Manual removal plus a licensed parasiticide |
| Flat disc-shaped creature moving on the skin | Fish louse (Argulus) | Manual removal plus a licensed parasiticide |
| Rapid or one-sided gill movement, gills held open | Gill flukes; gill damage from ammonia; low oxygen | Test water; examine gills |
Koi Fish Parasites
Parasites are the most common koi health problem, and most are invisible without a microscope. A skin scrape and gill biopsy read at 40–100× is how experienced keepers and vets identify them; treating blind wastes money and stresses fish.
- Ich (Ichthyophthirius multifiliis) — visible white grains. Only the free-swimming stage is treatable, so treatment must continue across the full life cycle, which shortens as water warms.
- Skin and gill flukes (Gyrodactylus, Dactylogyrus) — the most common koi parasite. Causes flashing, excess mucus, clamped fins and gill irritation. Needs a specific flukicide; salt does not clear them.
- Costia (Ichthyobodo) — a fast killer in cool water. Grey-blue film over the skin, lethargy, flashing.
- Chilodonella and Trichodina — ciliates causing excess mucus and flashing, usually in poor water or crowded conditions.
- Anchor worm (Lernaea) and fish lice (Argulus) — large enough to see. Remove individually with tweezers, treat the wound, and treat the pond to break the cycle.
Koi Fin Rot
Fin rot is a bacterial infection of tissue already damaged by poor water, fin nipping, or physical injury. Fins fray from the edge inward, often with a white or reddened margin; in advanced cases the erosion reaches the fin base.
- Fix the water. Test ammonia, nitrite and nitrate; do a 30% water change and repeat every other day.
- Add salt at 0.3% (about 3 kg per 1,000 litres / 2.5 lb per 100 gallons) for a supportive dip in the healing process, added gradually over 24 hours.
- Check for the cause — a sharp rock, a rough net, or a chasing male during spawning season.
- If erosion continues past a week despite clean water, move to a proprietary antibacterial treatment, or a vet-prescribed antibiotic for a single valuable fish.
Healed fins regrow with a clear or slightly deformed margin. Regrowth is slow — weeks to months — and slower in cold water.
Koi White Spots
Not every white spot is ich. Sort them by location and season:
| Appearance | Likely cause |
|---|---|
| Fine white grains scattered over body and fins, fish flashing | Ich (white spot disease) |
| White bumps only on gill covers and pectoral fin rays, spring, mature males | Breeding tubercles — normal, not a disease |
| Waxy white or grey raised patches, cooler months, no other symptoms | Carp pox — a herpesvirus growth, usually harmless and seasonal |
| Fluffy white tufts on a wound | Fungus, secondary to injury |
| Grey-white film over large areas | Excess mucus from parasites or chemical irritation |
Flashing, Lying on the Bottom, Not Eating
Flashing — rubbing against the pond floor or ornaments — means irritation. Two causes dominate: parasites, and water chemistry (ammonia, nitrite, chlorine or chloramine in untreated top-up water). Test first, then look for parasites. Occasional single flashes are normal; repeated flashing by several fish is not.
Koi lying on the bottom is normal in winter, when koi enter torpor below about 50°F and rest for months. Outside cold weather it points at low oxygen, ammonia poisoning, heavy parasite load, or viral disease. Check dissolved oxygen and ammonia before anything else. A koi lying on its side, rather than resting upright, is a more serious sign — suspect swim bladder problems or advanced systemic infection.
Not eating follows the same triage: temperature, then water quality, then recent changes, then disease. Details are in the koi feeding guide.
Koi Herpes Virus and Other Viral Disease
Koi herpes virus (KHV) is the disease that ends ponds. It becomes active between roughly 64 and 81°F, kills a high proportion of infected fish within days, and survivors remain lifelong carriers. Signs include sunken eyes, patchy or necrotic gills, lethargy and rapid mortality across the pond. There is no cure. In several countries it is notifiable, and infected stock must not be moved or sold.
Spring viraemia of carp (SVC) appears in cool spring water, with haemorrhaging, bloated abdomen and erratic swimming. It is also notifiable in many jurisdictions.
The only meaningful protection for both is quarantine: keep every new koi in a separate system at 65–75°F for three to four weeks, watch it, and never add fish straight from a shop bag into the pond.
Prevention and the Quarantine Routine
- Quarantine all new koi for 3–4 weeks at 65–75°F in a filtered, aerated tank.
- Test pond water weekly, and always after adding fish or a big rain event.
- Change 10–20% of the water weekly rather than waiting for a problem.
- Keep stocking density low — 500 gallons per adult koi.
- Never share nets, bowls or hands between quarantine and main pond without disinfecting.
- Keep a treatment kit ready: pond salt, a dechlorinator, a flukicide, a broad antibacterial, and a liquid test kit.
- Watch feeding time every day. Koi that miss a meal are telling you something days before symptoms show.
Frequently Asked Questions
Why is my koi flashing and rubbing on the bottom?
Flashing means skin irritation. The two dominant causes are parasites - usually flukes, costia or trichodina - and water chemistry problems such as ammonia, nitrite, or chlorine in untreated top-up water. Test the water first, then have a skin scrape examined under a microscope.
What are the white spots on my koi?
Fine white grains scattered over the body and fins with flashing indicate ich. White bumps confined to the gill covers and pectoral fins of a mature male in spring are breeding tubercles and are normal. Waxy grey-white raised patches in cool weather are usually carp pox, which is largely harmless.
Why is my koi lying on the bottom of the pond?
Below about 50F this is normal winter torpor. In warmer water it points at low dissolved oxygen, ammonia poisoning, a heavy parasite load or viral disease. Check oxygen and ammonia first. A koi lying on its side rather than resting upright is more serious.
How do I treat koi fin rot?
Fin rot is bacterial and secondary to poor water or injury. Do 30% water changes every other day, add pond salt gradually to 0.3%, and find the physical cause. If the erosion continues after a week in clean water, move to a proprietary antibacterial treatment or ask a vet about antibiotics.
What is koi herpes virus?
KHV is a viral disease active between roughly 64 and 81F that kills a high proportion of infected koi within days, with survivors remaining lifelong carriers. Signs include sunken eyes, patchy gills and rapid mortality. There is no cure, it is notifiable in several countries, and quarantining new fish is the only real protection.
Do I need to quarantine new koi?
Yes. Keep every new koi in a separate filtered, aerated system at 65-75F for three to four weeks. Most catastrophic pond losses trace back to a single fish added straight from a shop bag.