Common Koi Diseases

Cause

Bacterial infection, often triggered by poor water quality or injury

Symptoms

Fins appear ragged, frayed, or disintegrating at the edges. May see redness at the base of fins. In advanced cases the fin tissue recedes toward the body.

Treatment

Improve water quality first — this is almost always the root cause. Do a 25% water change. Treat with an antibacterial product such as Melafix or a veterinary antibiotic like Kanamycin for severe cases. Remove any sharp decorations that could be causing injury. Fins will regrow once the infection clears and water quality is maintained.

Cause

Parasitic infection by Ichthyophthirius multifiliis

Symptoms

Small white spots resembling grains of salt scattered across the body and fins. Fish may flash (rub against surfaces) and show clamped fins. Can be rapidly fatal if untreated in koi.

Treatment

Raise water temperature gradually to 82–86°F if possible to accelerate the parasite life cycle. Treat with an ich-specific medication such as Ich-X or salt at 0.3% concentration. Treat for the full recommended course even after spots disappear, as the parasite has stages that are not visible on the fish. Remove carbon from your filter during treatment as it will absorb the medication.

Cause

Parasitic crustacean (Lernaea) that embeds in the skin

Symptoms

Visible worm-like parasites protruding from the body, often with a forked tail visible. Red inflammation around the attachment point. Fish may flash or scratch against surfaces.

Treatment

Remove visible anchor worms carefully with tweezers, twisting gently to remove the entire parasite. Treat the wound site with an antiseptic such as iodine or potassium permanganate. Treat the entire pond with Dimilin or a lernaea-specific treatment to kill larvae in the water. Repeat treatment after 7–10 days to catch any newly hatched larvae.

Cause

Bacterial infection (often Aeromonas), usually entering through wounds or after stress

Symptoms

Open sores or craters on the body, often red-edged with white tissue visible at the center. Can appear suddenly and worsen rapidly. Fish may be lethargic and stop eating.

Treatment

Ulcers require prompt treatment. Clean the wound by gently removing any dead tissue with a cotton swab and apply a topical antiseptic such as iodine directly to the wound. Improve water quality immediately. Severe cases require injection or oral antibiotics — consult a vet. Pond salt at 0.3% helps reduce osmotic stress. Catch early — ulcers that reach the muscle layer or organs have a poor prognosis.

Cause

Bacterial infection affecting the kidneys, causing fluid accumulation

Symptoms

Pinecone appearance — scales standing out from the body like a pinecone due to fluid buildup beneath them. The fish may also appear bloated. Often fatal by the time symptoms are visible.

Treatment

Dropsy is one of the most difficult conditions to treat because it indicates severe internal organ damage. Isolate the affected fish immediately. Treat with antibiotics specifically targeting gram-negative bacteria such as Kanamycin or a veterinary prescription antibiotic. Add Epsom salt to the hospital tank at 1 teaspoon per 5 gallons to help draw out fluid. Unfortunately, the prognosis for full-blown dropsy is poor — early detection and treatment give the best chance of survival.

Cause

Microscopic parasitic flatworms (Gyrodactylus or Dactylogyrus)

Symptoms

Fish scratching and flashing, increased mucus production giving a grayish sheen to the skin, clamped fins, and rapid gill movement indicating gill flukes. Not visible to the naked eye — requires microscope confirmation for certainty.

Treatment

Treat with Praziquantel — it is the most effective and safest treatment for flukes in koi. Available as a pond treatment or medicated food. Two treatments 7 days apart are recommended as Prazi does not kill eggs. Salt at 0.3% can provide some relief but will not fully eliminate flukes. Flukes are extremely common and often present at low levels without causing visible symptoms — treatment is warranted when fish show clear signs of distress.

Cause

Infection, injury, genetic issue, or constipation

Symptoms

Fish swimming sideways, upside down, or struggling to maintain position in the water column. May float helplessly at the surface or sink to the bottom.

Treatment

First, stop feeding for 3–5 days — constipation is a surprisingly common cause and fasting often resolves it. If feeding resumes, offer a pea with the outer skin removed as a laxative. Check water quality and temperature. If the issue persists after fasting, bacterial infection of the swim bladder is possible — consult a vet. Genetic swim bladder issues in fancy koi varieties are unfortunately not treatable but affected fish can sometimes still live reasonable quality lives.