I have a 2 gallon hex tank with no filter and heater and my betta is doing fine... I normally change 80% of the water twice a week and sometimes once when I don't have time....I keep my new water in a barrel with and airstone and a heater(which is at 30 C), it's the same water i use in my main tank...
) Gravel is not that expensive. I have it and I don;t change the water for a month. The water I use is *cough* pure water bought from stores at a quater a gallon. Enough for two times of my tank. Please don;t think I hate my fish, Bettas are very hardy fishes. They live in drains and rivers back home in Malaysia. Really, no fuss is needed.