Did the freshwater dip show anything? I had wanted a powder blue tang, and after seeing a large one at petco for a good price, jumped at it, and without quarantine, added it to my 125g. Scratched a little, but seemed fine for the first couple of days, but on the third day I noticed rapid breathing and sluggish movement, and started noticing tiny translucent worm-like spots on its eyes. Turned out to be flukes. During the last few hours, the tang looked a lot like yours, laying on its side, so unresponsive that I could pick the poor thing up with my hand. Prazi-pro took care of the tank with several treatments, minus losing the beautiful tang and a flame angel. It may be unrelated to your case, but worth a check, because if it is a parasite you’ll need to treat the tank. Good luck!