I currently have have only 2 splendens, Cygnet, a Combtail I've had in a 5 gallon Walstad for about 4 years now (the age at which I lost my [sold-as-Half-Moon] hyperactive Silk, my 1st-ever male betta, who had a 15g, and Confetti, a Crowntail [who initially couldn't swim, having no fin membranes, and was never very active even when they mostly grew in] my 2nd-ever male betta, who had a 10g much of his life, though he wound up in a 5g in his last year)and a small Crowntail, who I recently bought because he reminded me of a fish I'd had and looked so terribly unhappy, and who is in a little 3-gallon in-a-pinch planted tank squeezed in under the q tank and looking much prettier and livelier than he did in the unheated little container in the store. (Duh, lol.)
Personally, I'd prefer to have them in larger tanks with more swim room and interest, but the plants make a big difference and give them something to swim through and around, a favorite betta activity.
But I've been greedy about more fish & more tanks, The Great Tank Reshuffle to make room which was supposed to take place a couple of year ago never has yet, and due to my father's recent death and the problems suffered by my mother (who I care for) since, we may be moving in the spring or summer so...
I'd say, if you're settled and you can manage something bigger than a 5g, go for it, at least I would.
They certainly can seem OK in a smaller tank, depending on the fish's character and activity level, and your circumstances.
But, as in the examples given, I'd never have put my non-stop-go Silk in anything smaller than a 15 and would not have downgraded Confetti to a smaller tank had it not seemed to suit his condition.
The new little one is better off than he was, and might have been, which is about all I can say about that, except that he seems happy and looks much better, and appears to enjoy investigating his tiny tank and the plants and rocks within.
Bettas are a group of distinct individuals, and, personally, I think that's why we love them so much and always want more...
