Hi, sorry I didn't answer earlier, I thought someone else would know more about koi treatments. I don't know about the most effective treatment for argulus, but a shop with a lot of good quality koi products should have a range of treatments. You can also pick them off the fish very carefully, but that may be difficult as it involves taking the fish out of the pond. If you do that, there are treatments to apply to the area they have been removed from to reduce the risk of infection. A koi expert would know how to sedate the fish for removing the argulus. It would be better to use a treatment than to pick them off, but I don't know what is the name of the best treatment.

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Lucia