How small of a tank?
Betta's are tropical fish, and should have a heater maintaining tank temperature at 78 F. They should also live in a filtered tank (especially if it is realy small). Very small tanks should have daily water changes to keep the inhabitants healthy.
I would recommend you buy at least a 5 gallon. They don't take up much more room than one of those micro aquariums, and in the end your Betta will be much happier. They also require a lot less maintenance, and are much nicer to look at.
Try posting this in the Betta section, or read their pinned topics (I believe there are a couple).