Most koi health problems are caught late because the signs are subtle and keepers wait to see if the fish 'gets better on its own'. Koi rarely do. Early identification is the difference between a treatable infection and a loss. This guide covers the most common issues and the protocols our quarantine team uses.
Behavioural Warning Signs
Before visible physical symptoms appear, behaviour changes first. Watch for: fish hanging near the surface in still water (oxygen stress or gill damage), flashing (rubbing against walls or substrate — early parasite irritation), isolation from the group, clamped fins held tight to the body, loss of appetite for more than two consecutive feedings, and unusual posture such as head-tilting or tail-hanging. Any one of these warrants a water test and closer observation.
Bacterial Infections — Aeromonas and Pseudomonas
The most common bacterial pathogens. Presents as ulcers (open red sores, often with white or yellow centres), red streaking on fins or body, fin rot with ragged edges, or raised scales (dropsy in severe cases). Dropsy — a pine-cone appearance from scales protruding — indicates systemic infection and has a poor prognosis. Ulcers: isolate the fish, clean the wound under anaesthetic (clove oil at 50 mg/L), apply Orabase or Propolis gel, and treat with antibiotic bath (potassium permanganate or prescribed antibiotic). Early-stage fin rot responds to salt (0.3%) and improved water quality.
Parasites — Anchor Worm and Fish Lice (Argulus)
Both are visible to the naked eye. Anchor worm (Lernaea) appears as thin white or green threads embedded in the skin, often with a red inflamed crater around the attachment point. Fish lice (Argulus) are flat, disc-shaped, translucent parasites up to 1 cm that move around the body. Remove anchor worms individually with fine tweezers — grip as close to the skin as possible and pull cleanly. Treat the pond with diflubenzuron (Dimilin) to kill larvae in the water. Argulus requires a full pond treatment; potassium permanganate dips or diflubenzuron are effective.
Gill Flukes (Dactylogyrus)
Microscopic flatworm parasites that attach to the gills. Primary symptom is respiratory distress — fish gasping at the surface despite adequate oxygenation, flared operculae (gill covers), and rapid gill movement. Requires microscopic gill scraping to confirm. Treat with formalin/malachite green solution (commercial products: Kusuri Fluke-M, Tetra FMG Mixture) or salt dips (30g/L for 10–15 minutes, watched closely). Flukes have eggs that survive treatment — repeat treatment at 7-day intervals to catch newly hatched larvae.
Fungal Infections
White or grey fluffy growth on skin, fins, or around wounds. Almost always secondary — fungus colonises tissue already damaged by injury, bacterial infection, or physical trauma. Treat the underlying cause first. Topical treatment: malachite green swab to the affected area. Bath treatment: methylene blue or commercial antifungal products. Salt at 0.3% suppresses fungal growth and supports healing.
Quarantine Protocol
All new fish should spend a minimum of 3–4 weeks in a separate quarantine tank before entering the main pond. Run the quarantine system at 18–20°C with good aeration. Observe daily. Treat prophylactically for parasites at the end of week one with a prazi (praziquantel) treatment to address flukes that may not yet be showing symptoms. Salt at 0.1–0.2% throughout quarantine reduces stress. Never skip quarantine — one infected new arrival can crash a healthy, established pond.