External parasites
Fish leeches
Fish leeches are segmented external parasites. Not every worm in a pond is a fish parasite.
Confirm the attachment
A leech attached to a fish is a different finding from a free-living worm in filter debris. Photograph its location and movement for identification.
Protect the skin
Do not tear attached animals off or apply household chemicals. Ask a professional about safe management and wound assessment.
Look around the system
Record any affected fish and recent plants or stock. The environmental stage and recurrence risk need attention as well as the visible animal.
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.