I would have ot disagree with this thread. I agree that IDEALLY 5-10 gallons (the bigger the better) is best. However you can have them in a smaller vase or tank without a filter if you change the water frequently. I have had my betta for over a year in a vase with a live plant (fish plant, not house plant) and I change the water once - twice per week and he is doing just fine. I feed him a variety of blood worms/brine srimp/betta pellets once a day, and there is still a good amount of room for him to swim around and flare his fins up at his mirror. He had pretty bad fin rot when I got him but his fins look great now.