if the tank is bigger than 2 gallons, then you can get a heater (a junior heater for 2-5 gallon tanks can be found at wal-mart for under $7) That heater I mentioned isn't automatic though, so it isn't good to keep it on overnight. The best thing to do to keep the small tank warm at night is keep the room it is in warm and cover it with a towel.
Betta's can tolerate a wide range of water temps, but they are tropical fish and prefer warmer water. I'm going to keep my tanks around 78-80 degrees, but right now it's around 82-84 while the tank cycles.
