I am pretty sure you cannot get a heater for a 1 gallon, however, you can buy a heater for the 2.5 gallon at walmart for like 7 bucks. I would advise in getting a 2.5 gallon tank and getting a heater because if you have any problems, which you eventually probably will as the colder temps cause stress to the betta, you wont be able to raise the temp. I have been trying to treat a betta with ich in a 1 gallon and it is almost impossible. Just my .02