I wouldn't put a heating pad on it, unless the tank is raised enough that there will be airflow underneath and around the pad. I used a reptile heating pad for a 5 gallon tank for a while, and it would get the tank too hot, even with airflow. Instead, look into a clip on light (they sell them for the smaller tanks, I think they're called petpals?). The light will keep the tank warm, but you'll want to monitor it and make sure it doesn't get too warm. Doesn't take much to warm up such a small volume of water. I have a betta in a 2.5, and the light in the hood will get the tank up to 83F easily, if it's on for more than about 8 hours.