Internal parasites
Black spot and encysted trematodes
Some digenean trematodes form cysts in fish tissues. A new dark spot is not automatically a parasite.
Colour is not enough
Normal koi and goldfish pigmentation changes can look different over time. Encysted parasites require identification, rather than treatment from a photograph.
Life cycles can be complex
Digeneans may involve more than one host. A bath treatment chosen for external protozoa may not address an encysted stage.
Ask the right question
Show the examiner dated photographs and describe whether spots are raised, spreading or associated with illness. Protect wildlife humanely; do not use poisons around the pond.
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.