My betta is in a planted 5-6g hex and I would never keep a betta in a smaller tank after seeing him in there! He 'patrols' the tank, blows enormous bubble nests and has two or three favourite 'hang-out' places which he seems to switch between depending on the time of day. Since the tank is planted and he has a couple of rock caves etc it's not like he has a huge amount of open water around him though.