I have two bettas in a 10 gallon tank. Was told I only needed one of these heaters. Not so. I purchased two just in case. I needed to use the second one as the side the heaters was not on, the temperature seemed to drop on real cosd nights. Put the second heater in the tank. Sometimes one is lit sometimes the both are lit. I also noted that when one only was lit it could be on either side of the divided. You certainly need two in a divided 10 gallon tank. They work great keeping the water at just the right temperature.