I have two of my bettas in a half gallon each....only thing is they are two and four months old. My other betta is in a one gallon and was recently in a half gallon and he prefers the half to the one.... I have a feeling it is because of the shape the one is tall and he doesn't seem to like that but the half gallon is wider than it is tall and he was bubblenesting like crazy. Needless to say i am looking for a new one gallon that is wider than it is tall because he seems to really be much happier that way.