External parasites
Costia: Ichthyobodo
Costia is the familiar keeper name for Ichthyobodo, a very small flagellate found on skin and gills.
Microscope clues
It is much smaller than many familiar parasites and can show a flickering or corkscrew-like movement in a fresh wet mount. Excess mucus may accompany infection. Careful lighting and focus matter.
A useful next step
Have a competent examiner review fresh samples alongside water results and breathing behaviour. An unclear video or a single negative scrape is not enough to settle a continuing problem.
Before treatment
Tell the adviser about every species present and anything already added. Ask when to recheck, rather than treating repeatedly because a fish still flashes.
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.