Bettas require a stable, warm tank temperature between 78-82 degrees Fahrenheit. This temperature should not vary more than a degree or two from night to day. It is important to keep your betta warm enough, otherwise, you may find that your betta becomes lethargic and prone to illness.
Since temperature plays such an important role in your betta’s health, a thermometer is essential to have for your betta’s tank. There are several types of thermometers: stick-on liquid crystal strips, or floating and submersible alcohol-based glass thermometers. While the liquid crystal stick-on type is beneficial to have, these are placed on the outside of the tank and may not be as accurate as a floating or submersible thermometer that is actually placed in the tank water. You can also use a hand-held thermometer to take a sample of tank water and measure the temperature. This device is particularly useful to make sure new water is at the correct temperature during water changes.
Heating a tank under 1-gallon with a tank heater is not recommended, but other options such as waterproof heating pads can be used. One of the downfalls of using a heating pad that you place beneath your tank is that it stays “always on” and will keep heating until you turn it off. If the air temperature is stable, however, this method will usually work if you place a towel between the heating pad and bottom of the tank. Heating pads get hot and maybe a fire hazard, so test your setup to make sure it appears safe and use at your own risk.
It has been suggested that using a 25-watt adjustable heater can be safe in tanks as small as 1-gallon if keeping a consistent, warm temperature is hard for you to achieve otherwise. Since this “technically” is not recommended, use it at your own risk and be sure to monitor the temperature stability of the tank on a regular basis. Make sure the temperature is correct and stable before adding the fish to the tank as it is never wise to mess with the temperature by more than a few degrees with fish in the water. If you do have to alter the temperature, do it gradually (1 degree each time) over the course of a few hours.
A 2-gallon tank can easily be heated with a 25-watt tank heater that is well-monitored. Anything above a 5-gallon tank can be safely heated; just be sure to monitor for occasional temperature spikes, especially during changes in seasons.