A good size heater would be 10-15 watts. Most people here recommend the Marineland stealth heaters, but I know in Canada I can't find any that are smaller than 25 watts. Adjustable heaters are better than preset ones.
I *personally* think that having a filter in a 2.5 Gal is a waste of time, as you'd have to change the water 3 times a week, one of them being 100% anyway.
As for food, some bettas are picky eaters, so you'll just have to experiment. All I know is that you should only give them bloodworms as a treat (especially freeze-dried ones) because if fed too much, it will cause bloating!
I think Hikari Gold is the cream of the crop though :D
Last edited by bahamut285; 04-04-2011 at 09:42 PM.