I have a 40 gallon as well and use a 150 watt, thinking about getting a 100 watt instead but if you get the Flukers 8.5 dome with a dimmer on amazon it helps control the temperature of the 150 watt one, it gets up to like 100 degrees but the dimmer helps keep it around 88-90!! Also make sure you have a digital thermometer, or an AcuRite to properly measure temps
Sent from my iPhone using Tapatalk