I apologize that no one has responded to your question sooner. Different experts come online at various times. I just came online and saw your question. My name is ***** ***** I’m a biologist with a special interest in fish and pond keeping.
Koi can end up with parasites in many different ways. They can be brought in on water plants. Insects and even birds visiting the pond can introduce parasites. New fish brought into the pond can be infested. Even older fish can carry parasites for months or years before any symptoms develop.
There are many different kinds of parasites. Some attach tot he skin, some infest the gills. Others are internal. Treatment varies, depending on the type of parasite. If you havea parasite problem with your koi, let me know what kind of pest, and I can help with treatment.
Hello again. Do you need any additional information?