Well said, Oldman, well said.
Umm....
Oh, right! Yeah, this is what I was going to say: First of all, STOP YELLING. Quit arguing with each other, and try to give the OP the best advice possible. Aside from that....
I kept Mr. Wimpy Fish in a 2.5-gallon which was non-heated, AND, non-filtered; and I only moved him out when he had some weird growth parasite-thingy in his gill. Once I moved him to a heated tank, well.... The thing just.... Well, it disappeared.

Other than that, he would've lived in it for quite some time. However, I found myself wanting another Betta -- so I divided the 5.5-gallon tank that he lived in then, and turned it into 2 2.5-gallon sections. I must say, 2.5-gallons is *quite* a LOT of a space for a Betta. Now -- because of the internal filter (I looooove Tetra Whispers <3), it's true that Libra doesn't have much swimming space. He doesn't *MIND*. He enjoys either napping curled up around the stalks of his plant (green cabomba), or, when he's not doing that, he's checking out my room, looking at me, or, when the lights are off, flaring at Mr. Wimpy Fish. Naughty boys. XD Anywhoo.... I do not see what your problem with keeping Bettas in 2-2.5-gallon tanks is. It works for me; it will continue to work for me -- and, I must say, this 2.5-gallon tank was what started all this with the bowls and whatnot in my house. ^^; Now, my Mom uses it for floating candles (no fish

), but.... Still. ^^;