I had a betta in a fishbowl who lived to nearly four years. His name was Blaze and he was the only fish I ever cried over losing. He died (wouldn't you know) shortly after an occasion when I asked my brother to feed him.
I don't know much about "fish", but I do know a scrap about bettas.
The
average male betta shouldn't be kept with anything other than himself. I speak of bettas you get at Walmart and low-grade petstores. They bite eachother, they bite their girlfriends, and they bite their unrelated tankmates. Some males can be housed with other creatures (even done it a few times), females of different numbers, and even other males. Like mentioned above, I think it depends on their habits and how they're raised. No small fish has a particularily good memory, but they can become habituated to a certain lifestyle over time, and male bettas in particular are very curious about unfamiliar occurances, and this may lead them to display instinctive aggression. I've never had issues with housing females together. They are my favorite aquatic creature, and I suspect they have atleast a little bit of intelligence. I have one now, and I've had her for a month at the most, and she's already completely into my routine. She knows when it's time to eat, and she will finger feed. If I take too long to bring the food near the water, she'll jump out and snap at it. I just love her.
I tried
once to permanently house a pair of bettas together when I was new to them. It was a hex tank from Walmart, but I don't remember how big it was. It was atleast 5 gallons. The bettas were standard from Walmart; one cobalt-blue male named Jericho and one melon-finned brown female named Olivia. Olivia was not carrying eggs, and Jericho had no bubble nest. Nevertheless, Jericho tried relentlessly to have his way with her, and he roughed her up quite a bit and even tore one of her fins. Once it got too rugged, I took Olivia out and Jericho subsequently tried to leap out of the tank, and eventually succeeded when I wasn't around. He could have been out of the water for as much as an hour. All I can say is, thank God for the labirynth organ.
Fish drama.