If you’ve spotted white, cotton-like fuzz on your koi, you’re almost certainly looking at fungus — and while it looks alarming, it’s very treatable once you understand what it is and, more importantly, why it showed up. The single most important thing to know about koi fungus is this: it’s almost always a secondary problem. Treat only the fuzz and it comes back. Treat the underlying cause too, and it clears for good.
What Koi Fungus Looks Like
The most common koi fungus is Saprolegnia, often called cotton-wool disease. It appears as white or gray cottony tufts on the body, fins, gills, or mouth. As it grows, it can look green or brown when algae colonizes the growth. It usually starts at a single spot — a scrape, an old ulcer, or an injured fin — and spreads outward from there.
Fungus Is Almost Always Secondary
Healthy koi with an intact slime coat are highly resistant to fungus. Saprolegnia can’t easily take hold on undamaged tissue — it colonizes tissue that has already been compromised by:
- Injury or physical damage (netting, rough surfaces, spawning)
- Parasites that damage the skin and strip the slime coat
- Poor water quality and chronic stress
- Cold water and a suppressed immune system
This is why treating the fungus alone so often fails: if a parasite or a wound opened the door, the fungus just comes back until you close that door. Always ask what let this happen, not just how do I kill the fuzz.
Don’t Confuse Fungus With Columnaris
One of the most common mistakes is treating a bacterial infection as if it were fungus. Columnaris (Flavobacterium columnare) also produces white, cottony-looking patches, but it’s a bacterial infection — it often appears around the mouth (“mouth rot”) or as a saddle-shaped patch across the back, and it moves fast. Columnaris will not respond to antifungal treatment; it needs an antibacterial instead. If the “fungus” is around the mouth, spreading rapidly, or looks more like a flat film than a fuzzy tuft, treat it as columnaris. Our koi physical symptoms guide walks through telling these apart.
How to Treat Koi Fungus
1. Test and correct water quality first
Before treating anything, test your water and fix any ammonia or nitrite. Stable water is the foundation of recovery — fungus struggles to spread on a fish that isn’t already stressed.
2. Address the root cause
If parasites are involved — and they very often are — clear them, or the fungus will keep returning. Browse koi parasite treatments to knock down the parasite pressure that damaged the fish in the first place. If it started from an injury, protect and monitor that wound.
3. Treat the fungus
For active fungal growth, Critter Quitter is formulated to treat fungal growths on skin, fins, and gills alongside common parasites, making it a strong choice when both are present. Purple Magic (potassium permanganate) also helps by oxidizing organic material and cleaning the fish and pond, reducing the fungal and bacterial load in the water. For isolated spots on a single fish, a careful topical application of iodine can be used, and a low level of salt supports healing.
4. If it’s columnaris, switch to an antibacterial
If the symptoms point to columnaris rather than true fungus, use Bacterial Blast (or medicated feed for internal cases) instead of an antifungal.
Preventing Koi Fungus
Because fungus is opportunistic, prevention is simply good husbandry: keep water stable, keep parasite pressure low, handle fish gently with wet hands and soft nets, support a strong slime coat, and quarantine new arrivals. A koi with a healthy slime coat and a stable environment rarely develops fungus in the first place.
Seeing cotton-wool fungus right now? Treat active fungus with Critter Quitter, clear the parasite pressure behind it with our koi parasite treatments, and if it looks more like columnaris, use our bacterial treatments. Not sure which it is? Book a koi health consultation.
Frequently Asked Questions
What does koi fungus look like?
Koi fungus (usually Saprolegnia) appears as white or gray cotton-wool tufts on the body, fins, gills, or mouth, often starting at a scrape or old wound and spreading outward.
What causes fungus on koi?
Fungus is almost always secondary. Saprolegnia colonizes tissue already weakened by injury, parasites, poor water quality, or handling — a healthy slime coat normally resists it.
How do I treat koi fungus?
Correct water quality, address the root cause (often parasites or a wound), and treat the fungus with an antifungal such as Critter Quitter. Isolated spots can be dabbed with iodine, and low-level salt supports healing.
Is it fungus or columnaris?
Columnaris is a bacterial infection that looks similar (cottony, often around the mouth or as a saddleback patch) and spreads quickly. It needs an antibacterial, not an antifungal, so identifying it correctly matters.
How do I prevent koi fungus?
Keep water stable, keep parasite pressure low, handle fish gently, support a strong slime coat, and quarantine new fish. Fungus rarely takes hold on a healthy, unstressed koi.
Written by Jason Michael, a 30-year aquaculture professional and founder of Krazy Koi Meds, with decades of hands-on experience treating koi and ornamental fish.
