If you use a fish bowl as a home for your fish, do your fish a favor and buy them a proper tank. They are simply terrible homes for any fish. I don’t know why pet stores and LFS (Local Fish Store) sell them to customers, they are not adequate. Not for a Betta, not for a Goldfish, not for anything. A Betta can live in low oxygen levels, but does it mean it’s the best? Well you can camp out on top of Mount Everest, where the oxygen levels are really low and let me know how it feels. You will survive, so surely it is ok, right? Wrong.
On top of the low Oxygen levels they are not capable of holding more than a gallon or two of water, often times much less. That is simply not enough space, nor is it large enough to use any type of proper filtration. The minimum tank size I would ever suggest is a 5 gallon tank, they are at least large enough to use real filters and provide some swimming space. You can’t keep much in one, a single Betta or some other tiny fish such as a pair of Guppies. This is why I usually say 10 gallon minimum tank size for most any fish, this way you’ve got a little extra space to add more. Trust me, you’ll eventually want to add a couple more fish. Happy Fish keeping and remember “Say No to Fish Bowls”.
Update – After some more research it appears a single Betta in a bowl isn’t that bad. You must keep the water changed and fresh though since without any filtering it will get gross much quicker. You also want to make sure you get a large bowl, something over 2 gallons at least. The tiny bowls <2 gallons simply do not provide what I would consider adequate swimming space. They can however live long lives if taken care of properly even in a filter-less bowl of water.