I have personally heated a 55g with a 50w heater that was preset to 79deg....now I must also say that the heater stayed on all the time and I had a maxijet900 blowing 200+gph on it to circulate the heat. If you dont have adiquate circulation i would recommend a larger heater...prob a 100w. Circulating water around it and the tank will vastly improve its performance.