Ooooh. I know that tank. I made the mistake of buying that one when I first brought home my first betta. Let's just say he outgrew it fairly quickly. A minimum of 1 gallon (though some would say 2gal +) is usually good for bettas. and if I remember right, that particular tank is about a quart. On another note, the plastic plant can nick your betta's fins. I would strongly recommend getting larger tanks for your bettas in order for them to thrive. my 1 gallon tanks I bought for about $9 at wal-mart and my 2.5 was $15 at petsmart.
I hate to sit here and whine about your tank size, but I figured the info might be useful. There are many other members much more experienced than myself that could give you much more information. Never be afraid to ask questions!