Ambient room temp makes a difference. I keep my house between 60-68. I ran 300W heaters in everything but the 20, and it had a 200W. The standard recommendation is for a heater that is 3-5 times the volume of water (55 x 3 = 165, x 5 = 275), so something between 150 - 300 watts is appropriate. As long as the heater is capable of keeping the tank at the appropriate temperature and sized to fit in the tank, it's okay. All heaters recently produced are supposed to fail in the off position--if you have ones that are older, IMO, it's worth replacing them.