I kept a purple tang in a 4 ft 120 for a decade or so. It was peaceful and happy as a clam...I thought.
When the time came to move the tank to a new house, the tang went to a friends 180 for temporary housing.
When the 120 was reset in my new house, I transferred all the other fish and I immediately noticed they were behaving differently. They were more active but also more calm.
Then I went to pick up my purple from his temporary home. As I watched him in the 180 it became obvious to me he was different as well. He was also much calmer. He seemed so graceful swimming in the larger tank.
The purple tang never returned to my 120. It was clear to me after this move that the 120 was too small, despite my thinking otherwise for 10 years.
Sometimes, we see things as we want, not as they are. I saw that a 120 was adequate for a decade, then I realized I was wrong. I wont ever keep a tang in a 4ft tank again. I'm not judging those who do, I have just decided I wont.
Just my thoughts on tang tank size...