Fungus

Signs

• Greyish-white, cotton-like growths on the skin, gills, eyes or fins
• Usually on damaged tissue or on eggs
• Can rarely invade deeper tissues

Cause

Water moulds (oomycetes), mainly Saprolegnia spp. (not true fungi). They infect fish eggs and injured or traumatised skin; fish kept below their preferred temperature range are especially susceptible.

What to do

First correct the environment: clean water, the right temperature for the species, and treat any other skin disease. Then a single treatment with potassium permanganate, formalin or hydrogen peroxide is often enough for external infection. Over-the-counter malachite-green products work in home aquaria but must never be used on food fish.

If a fish is badly ill, talk to an aquatic vet.

Prevention

Remove predisposing causes: wrong temperature, poor sanitation, excessive chemical treatments, and dead fish or decaying organic matter; avoid injuries.