I am dismayed to see a large number of people on ebay selling tiny tanks which are unfiltered and unheated and miniscule. They insist that these tiny things are fine for Bettas. I want to state categorically that they are NOT!!
Ideal set up is planted tank (not a bowl or jar).
Minimum volume for a single Betta is 1gallon, preferably 2 or 3 gallons.
Room temps are not adequate, unless the room temp is at tropical temps
Their ability to breathe air and therefore not asphyxiate in a small
amount of water often condems them to a lingering death from ammonia
poisoning in a vase or bowl.
So folks, please get those poor Bettas out of the vases or bowls!
They shouldn't be there!
Most of these fish can live for 5 years if kept well. I've had them for this length of time. But mine were always in a tank which was large enough for it to sawimm properly in, with clean filtered and heated water and not swimming about in filthy cold water in something little bigger than a jamjar. I have contacted the RSPCA with a view to them making it illegal to keep bettas in tiny unheated tanks in this country.
For people who admire the species as I do, and who might be interested in keeping one of them (male) or more (female), then here are some links to tropical fish information sites. If you are an animal lover, do not doom one of the most beautiful fish ever, to a miserable life in cold dirty water in a jam jar sized tank.