tang body condition

Hello, My new Lavender Tang is dying, and I'm trying to figure out what happened. Fish is a juvenile, made it through QT and was eating well, or so I thought. 30-day quarantine, treatment with PraziPro. After 30-day QT, has been in main display for about a week. All was fine last night, but...

Please see video. Recently acquired a mimic tang. Had it in QT initially but it seemed really stressed (20 gallon long). Was told by seller it had been in Copper x1 week and had been eating medicated food also for one week. I picked the best looking one weight wise but still thin to me. I...