I am no expert on Velvet, but my experience is that Copper seems to clear the free swimming rapidly and ones on fish drop off quite rapidly, only to be replaced by masses of free swimming, so if you clear the free swimming, you get rapid, one to two day visual improvement on the fish. This is my theory based on my own experience. I just recently had to treat a friends tank for Velvet, which I am comfortable calling this based on visual appearance alone. I saw complete clearing of massive fish infection in just 48 hours after initiating copper treatment. I will leave it to the experts to confirm or refute this.
sorry for the hijack and detour of this thread.