i allways said 3 but i think the new 2.5 they have come out with are pretty good and have a decent filter, ppl will say a 1 is more than big enough, well its not , yeah they will survive but they wont thrive which is our goal in keeping any animal. any way a 2.5 will make a nice home i like to keep mine with other things for entertainment for the betta, i usually use either african dwarf frogs or ghost shrimp or in bigger than 5 gallons i use panda corys or otos. but i think i already answered the question so ill just stop talking now