Check out this tiny heater, designed for tanks 5 gallons or less.
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=12087&N=2004+113767
I wish I'd known about that one years ago. My betta lived in a 2.5 for nearly 3 years, but I wonder if he would have lasted longer with a heater
Now I have a new betta in a divided 10 gal with a heater
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=12087&N=2004+113767
I wish I'd known about that one years ago. My betta lived in a 2.5 for nearly 3 years, but I wonder if he would have lasted longer with a heater
Now I have a new betta in a divided 10 gal with a heater