Happier in a smaller tank. Who knew?

OMG is Lir driving me crazy! He was originally in an unfiltered 3 gallon. He was an impulse purchase and had serious SBD. Once he healed from that, we began the tail biting issue. He gnawed on his tail. WE got him into a 2.5 filtered tank. He gnawed on his tail. I put him in a divided, filtered 10 gallon. The tail chomping really began. He had nearly totally gnawed off his back fin. I'd been wanting to try a planted tank, so we took the 2.5 gallon, planted it with some plants I had, baffled the heck out of the filter, and plopped him back into it. And, worried. Because he was suddenly not so active.
I'm beginning to think he is actually not all that active a fish naturally. He is a very awkward swimmer and does not swim at all like my other 4 fish. Today, I actually saw the beginnings of clear regrowth on the very tips of his fins. And it's only been 2 days!
I think he is happier in the 2.5 gallon tank. He doesn't look stressed, and he hasn't chomped on his tail in 2 whole days (this has never happened in the entire 3 months he's been with us)
I'm ordering more plants to add to his tank, and to redo the 10 gallon as a planted tank, and possibly add some plants to the 29 gallon in little pots until I get around to redoing that as a planted.
So I guess, some bettas really do prefer smaller tanks. Who knew?