a heater is a good idea, but 2.5 should be good for one betta. i keep one in a 3 gallon, and currently keep 2 in a 1 gallon, but they are under quarantine. it's all about proper water changes with small tanks. yours would be 1 50% and one 100% weekly with a good gravel siphoning on one of them.
There's a sticky at the top of this board on baffling filters to slow the current down that may help you keep it. But a filter isn't required you would just have to do more water changes.
Last edited by EvilVOG; 12-11-2011 at 11:50 PM.
Reason: TRIPLE JINX!