I’m currently using Zoo Med infrared 75 watt spot type bulbs on my 20 gallon long tanks. And have to turn the bulb dimmer almost all the way down as far as it will go in the summer months. So I’d give the 100watt a try. Petco and Petsmart will usually let you return it if you but the wrong size as long as you ask the manager in advance in my personal experience. Best of luck.