I would recommend keeping that betta by itself. As much as it may seems happy, betta are naturally solitary fish and doesn't like much of a water current. So just take caution with it...
No, theres no reason why I can't keep it in the 2.5g. Was just curious. I don't honestly expect this poor guy to live much longer anyways, as his previous owners were clueless, kept him is a terribly disgusting .25g, and didn't care. Today he seems to be doing pretty well. Lots of tail flailing, flaring, and dancing between bamboo shoots. Sadly theres still a lot of white on his face and hes still refusing to eat.
Thanks for the input guys. I'll let him live whats left of his life in peace.
Especially if he's been kept in such a little tank, he'd prefer some more solitude. I kept my male betta in a 55 gallon community tank for a while and he was fine, but he was used to strong currents because I had a good filter in his regular tank.
I put the same betta in the same community tank at another point in time and he killed two neon tetras... the tetras weren't in there the first time around. Bettas might just be drawn to attacking flashier fish? Maybe they looked like an easier target? Idk. Keep an eye out and see if everyone's getting along and to make sure the betta isn't stressed by the filter.
Agreed it depends on the individual fish, among other things. I've personally never had any luck mixing bettas with other fish. I think I've just had meanies though.