Two things here. First, the temperature setting on most heats is not always accurate with respect to the actual number. For example, my heater in the 33g tank is set to 21 C and maintains the tank at 24 C. This can work in reverse, so using a reliable thermometer you may need to adjust the setting.
Second, the temperature of the air in the room will affect the tank water temperature. This depends upon the heater setting (meaning the temperature the heater is actually keeping the water at). Most heaters need a room temperature that is not too far off the intended water temperature. Ten degrees F (roughly 5-6 C) or so is a good guide. I keep my tanks around 24-25C, and they are in a dedicated fish room. In winter I heat this room to 20 C day and night so the difference between air and tank water is not that great. If your room is getting down to say close to 3 C the heater may not be able to handle this difference.
