What do you do if your Betta fish tank is too small for a heater?

If your tank is too small for a heater it's too small for the Betta. The answer is simple. Get a bigger tank. A male Betta needs a heater, a filter, a light and living plants to be healthy. It grows to about 2 inches long so that means it needs at least 2 gallons of water ie a 3 gallon max capacity tank.