After cumpramine treatement and running carbon, blue tang got weird spots

[cristit]

I'm new in this hobby, started a new DT couple of months ago, got already Ich, got a QT and made Cupramine treatment for 14 days (with it's recommended dosage). All fish looked fine (tang included). Then made a 20% water change(yesterday) and started to run carbon filtration to remove copper. Today, blue tang has a lot of spots. Can be Ich again? Or is it something else? It is eating and swimming ok.
Attaching video...

Thanks!

 

[cristit]

 

[cristit]

In the last couple of hours looks even worse