Argulus, commonly called fish lice, are crustacean parasites that can attach to koi. Larger stages may be visible as flattened, rounded organisms on the fish. Skin irritation and damage can follow, but flashing or a skin mark alone is not enough to identify the parasite.
What to Look for
Observe the fish without prolonged chasing. Note visible attached organisms, irritated areas, repeated rubbing, and changes in swimming or appetite. Photograph what you can see and record whether other fish are affected. Species identification and the significance of an infestation may require an experienced examiner.
Why Removing One Louse May Not Solve It
Eggs can be laid on surfaces in the system, and unattached stages may remain in the water. Removing a visible adult does not address the whole life cycle. Development depends on conditions; a fixed month-long timetable is not a dependable treatment schedule. Skin damage also deserves assessment for secondary problems.
Plan Control with a Fish-Health Professional
Check water and oxygen while arranging diagnosis. A control plan must consider the fish, the environment, repeat monitoring, and product suitability. Avoid improvising pesticide or bath doses. If handling is needed, use trained help rather than repeated netting and scraping at an attachment site.
Keep It from Moving Between Systems
Use quarantine, dedicated nets and bowls, and traceable incoming batches. Do not share wet equipment from a suspect system. Follow the recommended follow-up checks after treatment rather than assuming that no visible adult means the infestation is gone. See the parasite comparison guide for similar problems.
Further reading: UF/IFAS: Argulus Infections.