How To Treat Crustacean (Visible) Parasites in Pond and Goldfish
This kind of medicine is pretty easy and the medicines are pretty easy to get.
CyroPro from Hikari is a chitin-synthesis-inhibitor like Dimilin, and what it does it interrupt the parasites' ability to make their own skin (exoskeleton) and that is fatal to the parasites. The anchor worm and fish lice parasite go through several "larval" phases before you see them on the fish and you kill those before they ever even show up on the fish. The 'adult' anchor worm simply dies because it can't even make a decent egg case.
This product Eco Labs LAWP seems to have the greatest economy, and is easy to get. Very safe to use, but it's no better or worse than Hikari's CyroPro pictured above.