External parasites
Chilodonella
Chilodonella is a microscopic ciliate associated with skin and gill problems. A grey coating is not a diagnosis by itself.
Identification clue
On fresh samples the organism can appear flattened, oval or kidney-shaped, with bands of cilia. It may be associated with excessive mucus. Use the linked identification guide for comparison.
What to record
Note whether rubbing, lethargy or breathing changes affect one fish or several. Record water tests and the time of sampling. Share clear microscope footage with a competent examiner.
Avoid the repeat-treatment trap
If signs persist, reassess the diagnosis and the water instead of changing chemicals by trial and error. Gill involvement needs professional assessment.
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.