Fish & shrimp health guide
Fungal Infection
Also called cotton wool disease, Saprolegnia, body fungus
Fluffy, cotton-wool tufts, usually white or grey, on an injury, the mouth, or on eggs.
TL;DR, Fungal Infection
True fungus shows as soft, fluffy cotton-wool tufts, and unlike the fast-spreading columnaris it grows slowly and almost always on damaged tissue, an old wound, a fin-rot site, or dead eggs. It is opportunistic, so the fish nearly always had an injury or was run down first. Improve the water, treat with an antifungal, and fix whatever caused the original damage. Healthy, unstressed fish in clean water rarely grow fungus.
What you see
White or grey fluffy, cotton-like growths on the skin, fins, mouth or over a wound. It looks softer and more three-dimensional than the flatter patches of columnaris, and it stays local rather than racing across the fish.
What causes it
- An existing injury or wound the fungus colonises
- Poor water quality and high organic waste
- A fish weakened by another illness or chronic stress
- Dead eggs in a spawning tank, which fungus spreads from
How to treat it
- Clean the water with a change and better maintenance.
- Treat with an antifungal remedy suited to the fish.
- Address the underlying injury or illness that let it start.
- In a fish-only tank, aquarium salt can help; not in a planted or shrimp tank.
- Remove dead eggs or organic matter feeding the fungus.
Treatment safety in a planted tank
Antifungal dyes like malachite green stain silicone and harm shrimp, snails and scaleless fish. Use an invert-safe antifungal or treat in quarantine, and never salt a planted display.
Prevent it
Keep water clean, treat injuries and fin rot early, and reduce stress. Fungus needs a foothold, so a fish with intact skin in good water rarely gives it one.
Easy to confuse with
Columnaris looks fungal but is bacterial, spreads fast and needs antibacterial treatment. If the cottony growth is racing across a healthy fish rather than sitting on a wound, treat it as columnaris, not fungus.
Species most at risk
Species that catch this most often, or that need a gentler treatment:
Frequently asked questions
Direct answers to the questions search engines and AI assistants surface most often about this species.
Is it fungus or columnaris?
Fungus grows slowly on a wound or dead tissue and stays put. Columnaris spreads fast across healthy skin. When in doubt on a fast-moving case, treat for columnaris.
Where does fungus come from?
Fungal spores are always in the tank. They only grow on damaged tissue or dead matter, so an injury or poor water is the real cause.
Can I use salt in my planted tank?
No. Salt harms most plants and inverts. Treat the fish in a separate quarantine tank if you want to use salt.