I believe some of the comments posted here have quite a tone animosity, even if unintended. It's MUCH more effective to politely state your opinion as an opinion, perhaps even an informed opinion with cited studies, then to absolutely say others are "WRONG!!!!" and are being cruel and/or foolish for thinking otherwise.
I've been keeping fish for almost 30 years, reef tank for almost 25...I was around before the Tang police, and am still here long after. There are no scientific studies that I know of that relate to tangs or butterflies in smaller (sub 100G) tanks that show them to have shorter life span or to behave out of character because of those confines. I've had a purple and a blue tang in an 80 gallon tank for over 15 years, the purple tang approaching 20. They are healthy and fat and exhibit similar behavior to tangs in much larger systems. Were I to do it over again, sure I'd use a larger tank, but to make the blanket statement that it's absolutely wrong to keep any butterfly in a tank under 125g, is, in my opinion, not true.
This is a false comparison, a dog is clearly and evidently, a sentient animal. Take a dog out of it's surroundings and put it someplace new, you can tell it's scared. Place a fish in a net, take it out of the ocean, put it in a store, net, bag, floating, Q tank...and then it's swimming and eating in 5 minutes. Of course this depends on species, but you get the point. Research in this area on fish is small and sketchy at best, and even if it turns out that fish do have some sort of higher function, it will in no way be comparable to that of a mammal. Note that this does not in any way mean that I am pro big fish in small tanks, only that it is not so black and white, and that perhaps, even though it is the internet, we should all treat one another respectfully.
While, like others, I do agree that 2.5' tank is probably not the best permanent environment for a butterfly, I don't believe stating that anything less than 125g is unacceptable. There are several other factors to consider, such as tank dimensions, amount of rock inside, and size of individual fish.