It could be velvet - your copper is just at the borderline. It could also be a disease not treatable with copper. It could be oxygen related. It could be another toxin. Also possible that your badge is not working correctly. Unfortunately, I can't see the pictures. Is there anything that looks like a bacterial issue (torn fins, reddish coloring?)Things have been going well with the quarantine until this week. Suddenly lost a clown tang, hippo tang, six line wrasse, and powder brown (fast breathing and spinning), scopas (fast breathing areas of discoloration) and Kole (some discoloration) tangs are looking rough. Coppersafe has been at a constant 2.0ppm verified by Hanna checker the whole time and ran 3 rounds of prazipro. Ammonia is 0 according to the ammonia alert badge. What could come out of the blue and kill/infect all the fish this quickly, bacterial, brook, bad badge and ammonia? (clownfish is fine, as are all angelfish)? I attached videos.
Though many people (in other threads) have stated that it's safe to use copper with prazipro - others have said it's not a good idea. (its unclear whether you used the 2 together) - also heavy aeration is needed with Prazipro.