External parasites
Trichodina: the rotating disc parasite
Trichodina are microscopic ciliates that can occur on freshwater fish, including koi and goldfish.
What an examiner looks for
A disc-like organism with a characteristic rotating movement and denticle ring can be seen on a wet mount. Seeing movement alone is not enough; debris and other organisms can be confusing.
Look beyond the slide
Review accumulated organic waste and maintenance as well as the parasite finding. Keep a record of numbers seen, sample location and the fish’s condition for the adviser.
Follow-up matters
Use an appropriate, confirmed treatment plan and arrange a recheck. Improvement in rubbing alone does not demonstrate that all causes of irritation were resolved.
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.