Koi Parasites Anchor Worms
Lernea elegans, the most common type of Anchor worm affecting Koi, is a real threat. They attach ventrally, they hold on for about 14 days, and they reproduce copiously. The wounds they create almost always infect with Ulcer disease bacteria, Aeromonas, and then you have two problems.
Treatment can be undertaken with Diflubenzuron
I do recommend removing any adults you see attached and swabbing the wounds with Iodine or mercurochrome but when you use Diflubenzuron, it's not necessary. I do also recommend also feeding an antibiotic food when you see Lernea to head off problems.