Bacteria & water moulds
Saprolegnia: water mould on fish
Saprolegnia is a water mould often described as fungus. It can affect fish and eggs.
What it can look like
White, grey or brown cotton-like material may appear on damaged areas. Other conditions can resemble it, so a fresh examination matters.
Find the underlying damage
A professional may examine a sample for hyphae and investigate injuries or other disease. Growth on a decomposing fish does not necessarily explain what killed it.
Plan treatment and recovery
Seek appropriate treatment for the confirmed problem and all species present. Keep water stable and arrange follow-up if tissue damage continues.
Sources & further reading
Original WakeyKoi wording, informed by the references below. Linked clinical resources explain the evidence and their scope; they have not reviewed or endorsed this website.
Related guides
Educational guidance cannot diagnose an individual fish. Breathing distress, severe injury or unexplained losses need prompt professional assessment.