External parasites
Argulus: fish lice
Fish lice are crustacean parasites, not insects. Larger stages can be visible on koi or goldfish.
Recognising a possible louse
Look for a flattened disc-like animal attached to or moving over the fish. Photograph it without repeatedly chasing the fish, and arrange identification.
The pond is part of the problem
Eggs and developing stages mean removing one visible adult may not finish the infestation. Ask for a plan that addresses the life cycle and the system.
Treatment choices are specific
Do not pull at the fish or improvise insecticides. Confirm the product is appropriate for all inhabitants and follow professional directions for follow-up.
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.