“Flukes” in goldfish doesn’t refer to an actual disease, but rather a parasite, which comes in two different varieties depending on whether it affects the body or the gills of the fish. Flukes are one of the most common types of aquarium and pond parasites, and one of the largest too, which can be seen with a very low magnification but still, not with the naked eye.
While there are two different types of fluke parasite- gill flukes and body flukes- treating them and eradicating them is undertaken in the same way.