Taken from
http://www.fishkeeping.co.uk/modules/zmagazine/article.php?articleid=46
Bettas are Anabatoids meaning they have a special Labyrinth organ allowing them to breathe air from the surface of the water. This means they can live in conditions, such as in rice paddies, stagnant ponds, ditches and slow flowing streams, which would most likely be fatal to other fish. Bettas must have access to the surface of the water to breathe. They can actually drown without this access. Studies have shown that a Betta with its Labyrinth organ removed, even in oxygen saturated water, will die.
Bettas are one of few fish, possibly the only type, which can be kept in very small tanks, bowls or even the infamous Betta Vase. This is due to their capacity to live in poorly oxygenated water and lack of activity. It is said that Bettas can be kept in bowls or cups of 4 litres or even 2 litres but many argue that that is too small - the Betta may survive but it will not thrive. Until recently it was common place for Bettas to be sold in a vase with a Peace Lilly on top. Owners were told the fish would eat the flower's root and that water changes were not necessary. Conversely, Bettas do not like very large tanks either, possibly due to the fish 'patrolling' their territory and becoming tired and stressed. Many owners, kindly liberating their fish to a larger tank, have found themselves moving their fish back into a smaller tank again as their fish was so unhappy in its new home! Ten to 12 gallons in big enough for a fish on its own although people have successfully kept Bettas in larger community tanks.
Tanks need to be heated to a constant 24-29 C (75-84 F). Water temperature is a key factor in keeping your fish happy and healthy. Any current caused by filtration should be minimal so as not to stress the fish, this will mimic their natural habitat. Also, Betta fish are not usually strong swimmers due to their heavy finnage. Bigger tanks need to be filtered but many keepers who have smaller tanks do not have filtration and keep the water clean by doing regular (usually daily or bi-daily) water changes of up to 50%.