My betta's home

I got my betta yesterday from Petsmart. :-D He is living in a cylindrical 3-gallon tank with a few leafty plants and, lol, a Spongebob house. I tested the tap water and used conditioners and those little fizzy pH pills for the water. I tested it and everything was fine. The temperature yesterday when he was first introduced was around 78, I think. And now it is about 72. :| My betta is actually very active compared to the ones in the store. He has lots of hiding places and plenty smooth stones and leaves to rest on. He just can't seem to stop moving, though! He swims around pretty fast and I was wondering if the enviroment might be too cold, or maybe not right for him. :-( I'm afraid of adding my heater, though, because the temperature is already so close to where it needs to be. I am new at this and a bit nervous. (I don't want to kill him!!!) Please help. :dunno: