I can view the video - it's not clear to me what's going on. the fish has obvious spots, but some of them are too large to be ich, these are "mucus plugs/cones" that tangs show when stressed. Very often, this mucus is combined with uncontrolled ich. However, 30 days of copper power would have resolved that. One idea might be that the copper power bottle you have is bad, but I'd rule that out because your copper reading is fine.
The fish is breathing way too fast. Ich doesn't cause that. Velvet does, but the copper power would have fixed that issue. Flukes cause that - Prazipro needs to be dosed multiple times for it to work (it only works for a day or so for each dose). Maybe the single dose didn't handle the gill flukes? Here is a post I made about proper prazi treatments:
Using Praziquantel (PZQ) to treat flukes Overview In the early 1990’s, a researcher at the New York Aquarium published a paper on using Praziquantel (PZQ) to help control flukes, a parasitic flatworm. Also known as monogenean trematodes, these fish parasites can cause significant mortality in...
www.reef2reef.com
The other thing to consider is that the rapid breathing isn't caused by velvet or flukes, but rather, with something wrong with the water. Is there good aeration? Not just circulation, but bubbles actually breaking the surface of the water for good gas exchange. Or, is there high ammonia, or some other water quality issue?