It would be perfect for a betta. Put some smooth gravel on the bottom for bacteria to grow on, add the heater and put in a plant. If you use a fake plant make sure it is silk or nylon so he won't rip his fins. If you want a live plant a java fern is probably the easiest choice for you, little light, little care and it seems to do fine. If you don't have a filter for this tank make sure to change the water a couple of times a week (partial, maybe 30%) to clean out the waste. Sometimes bettas like a small stream of bubbles as well. At least mine always seem to play in them.