SupaVerm
UPDATE:
A much safer treatment for flukes that will not kill goldfish is Aqua Prazi. Aqua Prazi will cost more, however, it will not put stress on your koi or kill your goldfish.
What It Is:
SupaVerm© is a white liquid. It's a suspension of Closantel and Mebendazole imported from Europe for use as a drench dewormer for sheep.
What It's Good At:
Applied to the water of ornamental Koi, SupaVerm© eliminates fluke (trematode) infestations within 3 days.
Pros:
SupaVerm© has been shown to be safe in single and double applications to eliminate Flukes without negative effect on Koi, plants, or the beneficial nitrogen-reducing bacteria in the filter. No water changes are indicated.
Cons:
THIS PRODUCT WILL KILL 100% OF TREATED GOLDFISH DEAD! IT IS NOT FOR GOLDFISH. IT IS FOR KOI.
There are other species of fish which are sensitive to SupaVerm© as well. Use in any other species of fish than ornamental Koi is sheer lunacy.
Dose:
Variable, based on experience and preference and whether the Supaverm is diluted. Normally: One milliliter of pure, per one hundred US gallons, applied on Day One and Day Two.
UPDATE:
A much safer treatment for flukes that will not kill goldfish is Aqua Prazi. Aqua Prazi will cost more, however, it will not put stress on your koi or kill your goldfish