When starts looking uncomfortable and starts pacing from left to right or even up and down ( "glass surfing" ) ive found freshwater fish do over the years when tank to small.
New to saltwater at 8 months is why input from freshwater fish experience but i would guess a stressed fish is a stressed fish either freshwater or saltwater.
But i see recommended size tank for these fosh is minumum 100 gallons but preferably 200 gallons so if not stressed then your tank could be ideal.
Ovbiously fish that like to swim like more width in a tank rather than height ( ovbiously not shallow like 6 inches high ha ha )
IMO after 8 years this fish has maxed out on size and a 6 foot long 180g tank should be fine. If it is showing signs of stress like Ying Yang said, then that is another story
I would say not so much for agression, hasn’t killed anyone, doesn’t really bother anyother fish, ocassionaly might chase a chromis when he turns but i wouldn’t say agression