Oral lesions (mouth sores) make it painful to eat and talk. Two of the most common recurrent oral lesions are fever blisters (also known as cold sores) and canker sores. Though similar, fever blisters and canker sores have important differences.
You could do a school of White Cloud minnows, they require cooler water, so you wouldn't need a heater. I think if you check the profiles, the ideal temp should be no lower than 67 degrees. Not positive.or example: my goldfish tank is fine in the summer, but my house gets so cold in the winter that I need to have a heater to keep it above 70F. Fish need a constant temperature which is one of the reasons we use heaters.
Betta need heat and filtration just like any other tropical fish, and this is very difficult to accomplish in a tank under 5 gallons. There are nano filters and heaters available for tanks as small as 5-gallons, and these are smart addition to your setup.