Buddy needs your help

[6 man reef]

Yellow tang in '1 year young' mixed reef. Noticed fish expelling gas a few weeks ago (may be normal or unrelated to current problem?).
Yesterday he had pop eye. Has been fighting with hippo for a night time hang out. No other fish in the system is ill. This morning he is struggling to keep himself from floating. Large fast fish in 120 gal reef with too many hiding spots to catch him. Can someone assist with a treatment.

Thank you in advance for any assistance
Mike

[JGoslee]

I would get him to a QT tank immediately.