Well... it depends. How small is the tank? IMO, a tank should be at least 2.5 gallons to hold a low wattage heater. Some have used the 7.5 watt heaters - although they only bring the temp up a few degrees from room temperature. A betta does best in water that is at least 74 - 80F. If you add a heater to a smaller tank - make sure you have a reliable thermometer to keep tabs on the temp that it doesn't ever get too hot.