Fish Lice (Argulus)
Argulus another crustacean parasite, round and up to 1cm wide. They have a sucker to hold on to the Koi with needle-like mouth parts which they stick into the Koi and inject a toxin. This causes intense irritation to the Koi and they scratch and jump and can cause bacterial infection.
If they infect the gills they cause severe damage and often death. Most antiparasite remedies will not kill fish lice, a strong chemical is needed which is not freely on sale. Ask a professional dealer or vet.