Fish Fungus Treatment: Cotton-Wool Growth Cured (UK)

Jaqline Smalling

August 25, 2026

Fungus shows up as fluffy, cotton-wool-like tufts on a fish’s body, fins or mouth — and while it looks alarming, it’s very treatable. The important thing to understand is that true fungus is almost always a secondary problem: it takes hold on fish already weakened by injury, stress or poor water quality. This UK guide covers how to treat fish fungus and stop it returning.

Here’s what to do.

Key Takeaways

Fish fungus looks like fluffy cotton-wool tufts on the body, fins or mouth.
It's almost always secondary — it attacks fish weakened by injury, stress or poor water.
Treat by fixing water quality first, then dosing an anti-fungal (removing carbon first).
Prevent it by keeping water clean, avoiding sharp decor, and treating injuries promptly.

Last reviewed: August 2026 · Written by Jaqline Smalling

Photo via Unsplash.

Recognising fungus

White or greyish cotton-wool tufts on the body, fins or mouth.
Often appears on a wound, damaged fin, or after another illness.
Distinct from ich (tiny hard white spots) and velvet (fine gold dust).

Treatment

Step 1 — Fix the water. Test and do a large water change; poor conditions are the usual trigger. See reducing ammonia.

Step 2 — Treat with an anti-fungal. Remove carbon first, then dose eSHa 2000 or a fungus & finrot treatment, which also covers the bacterial infections that often accompany fungus.

Step 3 — Isolate badly affected fish in a hospital tank if you need to treat one fish intensively.

Preventing it

Because fungus preys on weakened fish, prevention is about keeping fish strong: high water quality, no sharp decor that causes wounds, gentle handling, and careful acclimation. Treat injuries and other diseases promptly before fungus can take hold.

Frequently asked questions

What does fish fungus look like?

White or greyish cotton-wool-like tufts on the body, fins or mouth, often on a wound or after another illness. It's distinct from ich's hard white spots and velvet's fine gold dust.

How do you treat fungus on fish?

Fix water quality with a large water change, remove carbon from the filter, then dose an anti-fungal such as eSHa 2000. Treatments that also cover bacterial infection are ideal, as the two often occur together.

Why do my fish keep getting fungus?

Because fungus attacks fish already weakened by poor water quality, injury or stress. Recurring fungus points to an underlying husbandry issue — improve water quality and remove sharp decor.

— Jaqline Smalling, FishForever