I'm looking for a small heater for a 2.5 betta gallon tank. I can't believe the wattage difference in all these minI heaters, anywhere from 5 to 50. I can't decide which one I feel comfortable using. I would like it to be adjustable since its for a betta but also submersible because of the small tank size. Is there such a thing in a small heater? Any input would be great.