I hope it's not too late to answer this, but the smallest I would ever put a betta in is three gallons. I keep all of my bettas in at least five-gallon tanks though, because it is easier to keep the water clean and have good water parameters. You hear people say they are from really shallow water, but that shallow water is miles long and constantly flowing. You will also hear people say they don't need a heater, but you have to remember these are tropical fish and need temps of at LEAST 65 degrees or higher...
