Let me guess your tank is 6 foot? So I see these guys swim circles in corners more than in rectangular in that case length is infinite is it not? I have seen yellow tangs happy in 55 gallon corners
No actually. I had to down-size after an injury meant I couldn't keep up with the maintenance of a larger tank - so now my tank is not big enough for a tang.
No, length is not infinite - tangs will naturally swim in a line before turning, if they are swimming in circles this is not appropriate housing for them.
How do you know those tangs are happy?