I know it can be frustrating, but don't let it get u down. I was having the same problems, went thru 15 of 16 fish. Then I switched to a QT procedure that
@HotRocks uses and I'm 6 for 6 and have another 6 fish in QT right now.
Qt procedure
Match salinity with water fish came in, usually 1.019 to 1.020 and raise copper to 1 ppm with copper power.
Float bag for 30 minutes to temp acclimate and then release into QT tank, no salinity acclimation.
Start feeding with food soaked in GC and focus for 2 weeks.
Raise copper to 1.75 ppm over 3 days.
You can then transfer fish to sterile tank after 14 days or go for 30 days.
At the end of copper treatment you remove copper with water changes equally over a 100 percent water replacement over 4 to 5 days and also use carbon removal.
Then treat with prazi 2 times.
I get my fish from
@FishyBusinessAq with great success. I originally got my fish from my LFS and again, I lost 15 of 16 fish. Right now I have 2 tangs and 4 wrasses in QT using the above QT procedure. Good Luck, you will succeed!!!