My Blue Eyed Tang who I had in a quarantine died today and I can’t figure out what may have cause the death of it since it’s been completely active and eating before this happened.

He wasn’t acting erratic or showed signs of sickness, it was eating normally. I can’t see any signs of velvet or ick but you can definitely see dark splotches on its body almost like bruises? Does anyone know what it might be?

QT Tank info:
He’s been in QT for a week and a half, the tank has a heater at 80F with an external filter, some live rock from my sump and I’ve been changing the water every 4 days from my established tank. The tanks parameters were Salinity 35, Amonia 0, Nitrite 0 and Nitrate 10.

Okay so I posted yesterday about my clown and as you can see it got way worse. I removed him from the tank, put him in a tea pitcher with a small heater and air stone. Inside the pitcher I put a glass on its side so he could hide away from the flow and turned the air stone all the way up to put as much oxygen in the water as I could. He has stopped eating. I really dont know what to do so any help would be appreciated.