Argulus
Koi Fish Lice "Argulus" is a crustacean or branchiurian parasite most commonly encountered in koi ponds, but they are also found in aquaria. They are easily detected when they strike. Fish lice are greenish disc shaped organisms with suckers and small legs. Fish lice even have a pair of eye spots on the anterior end. They spend their time darting around in the water away from, and also directly on your koi. Fish lice lay their eggs in tubular structures on the glass and ornaments. Fish lice are not micrscopic, they can be seen on your koi unlike most koi parasites.
Fish lice can be very destructive to your koi.
Fish lice carry Aeromonas and other bacteria on their feeding stiletto and thus infect each koi they bite.
Treatment for fish lice is by the application of: Diflubenzuron.