6ft 180 is a good size for tangs. How many and what kind are they.
If your upgrading for the fish then make sure to get the size right the first time so you don't have to upgrade again.
Maybe somebody else can chime in, but for a pair of Blue Tangs you're looking at 180. Single is 125, but more than 1 in that size is dangerous.
Just get the biggest tank you can afford....buy a quality used aquarium if possible you will pay like have the cost of the orginial price and usually get many years of service out of it....maybe try your local area club....for used equipment
Personally, I disagree with this. IME more than one tang (not a sohal) can be kept in a six foot long 125. I have done this personally and seen it done successfully many times. But again, tank size is a very subjective thing.