Bacteria & water moulds
Fin rot, split fins and tail damage
Fin damage may start with trauma and develop infection. “Fin rot” describes a problem that still needs a cause.
Watch the edge
Compare dated photographs for spreading redness, tissue loss or healing. Normal transparent regrowth should not be confused with continued breakdown.
Remove the source of injury
Look for sharp décor, rough netting and aggression. Check water quality and whether a slow fancy goldfish is being chased or nipped.
Get help early
Worsening damage or involvement of the fin base warrants assessment. Do not trim damaged fins at home or guess an antibiotic.
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.