Hogwash, ionic copper dosed to 0.3 ppm will not hurt a tang. All my fish, including 5 tangs, went through an ionic copper treatment without a loss. And to this day, every one of those fish is fat, healthy and growing well, some of which I've had since I started 4 years ago.
Ionic copper is the best treatment hands down. Chelated copper and cupramine are easier on fish, but they are also easier on the parasite. I have used all formulas and found that crypt can make it though both chelated and cupramine treatments, oftentimes requiring a second 3-4 week treatment.
FWIW, most people use cupramine for two reasons, one, it's readily available in the US, given that Seachem is a staple in most pet stores and two, it's harder to screw up using it.