Why Does Your Koi Have White Spots, and How to Treat Them?

1 Causes and Treatment of White Spots on Koi

1. White Spot Disease (Ich)

Ich is a parasitic infection of koi skin, fins, and gills that feeds on the fish, spurs mucus, and forms white pustules. Infected koi go listless, move less, gasp at the surface, thin out, breathe hard, and rub on plants and gravel; they die within 5–10 days.

Treatment: the parasite can’t take heat, so raise the temperature to about 30°C to mature the spores so they drop off. Also isolate the koi and salt-bath it 1%–5% for days, or dip 2 ppm mercuric nitrate for 30 minutes.

2. Water Quality

Long gaps between water changes or overfeeding pollute the tank, enriching it so harmful bacteria and parasites bloom and compete for oxygen, causing spots.

Treatment: scoop some clean top water into a basin, move the koi there, scrub and disinfect the tank, refill with sunned water plus 1/4 of the old, restart filtration, then return the fish. Change water regularly—at least three times a week in summer, 1/4–1/3 each, not too much to avoid shock.

3. Fight Wounds

Large, active koi in dense stocking fight and get hurt; untreated in poor water, wounds rot and show surface white spots.

Treatment: net the injured koi away from bullies; if co-habiting, use a bigger tank. Keep it alone and quiet until healed, disinfect the wound with diluted potassium permanganate, and it recovers.

These are the common causes and treatments for white spots on koi; hope they help. This is for reference only—if it fails, ask an online vet.