One gallon is pretty small
I originally had my first Mr. Foosh in a tiny Betta Condo. They just laze around and seem listless. On the recommendation of my local pet store and a very knowledgeable young lady, I put Mr. Foosh II in an Eclipse 3. Tank is 3 US gallons, has an excellent in-hood filtration unit. Some people think that the Eclipse has too much current, but my little guy goes right into the current, kinda like an aquasizer!
Give him room to swim and you will be amazed at the difference. As to heater, put a thermometer on the tank before investing in a heater. My ambient room temperature is around 72 degrees, but the tank stays at an honest 80 degrees overnight and 82 - 84 during the day when the light is on.