they should be able to get some kind of partial or indirect sunlight, if you don't have a light for them.
do you have a thermometer? if your room is in the mid-70's, usually the water temperature is lower than the room temperature, so your water might be around 72* F.
bettas require warm water, best around 76-82* F. first, get a thermometer, then a heater if needed. if his water is in the right range, you should probably still get a heater for the colder months. also, lamps can be iffy, because they can cause the water temperature to fluctuate, which isn't good for bettas.