@mistress obviously has a lot more experience with your specific tank. Bettas are curious little guys! I use the sponge baffle on my 10 gals because my female VT would be up inside the bottle baffle in a heartbeat! I would agree that putting one betta per 2.5gals is best. Most experienced betta keepers reccomend nothing less in size. My expertise is the larger tanks and cycling, so I won't be much help beyond the cycling questions. However, if you do decide to upgrade to a 10gal, you'll probably want to go with a "kit". It includes the tank, hood, filter, heater, ect. If you buy them separately it can be pricey, and with a 10 gal you'll need to cycle it.