Internal parasites
Internal worms: what a thin fish can tell you
Weight loss does not prove a worm infection. Diet, competition and several illnesses can produce a similar appearance.
What may be investigated
Nematodes are roundworms with different life cycles; some use intermediate hosts. A professional may examine faeces or other appropriate samples rather than relying on a skin scrape.
Avoid routine deworming by guesswork
Choose a treatment only after identifying a relevant parasite and checking the fish can receive it safely. Aquarium worm medicines are not interchangeable.
Useful observations
Record appetite, body condition, the food offered and which fish actually obtain it. A short video of feeding can reveal competition that a still photograph misses.
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.