Assuming the heater has the capacity to heat at your room temperatures and considering the size of your tank, then a heater alone should be able to keep the temperature stable. If the problem is that the room gets so cold that even with the heater on, the tank continues to get colder and colder, then you need a second thermometer. The Jagers by Eheim are very precise and assuming they can heat up the tank at your specific room temperatures, they should be able to keep it steady. From what you tell me, I assume that your problem is that the room the tank is in gets so cold that the heater can't keep up. In that case you need a second heater. The benefit of a controller is only that it can act as a fail safe when it comes to operating a heater with thermostat like the Jager. In other words, all it does is allow electricity to pass through and power up the heater when the temperature hits a specfic low point and stops the electric current when the water hits a high point. Even with a controller, depending on where you set the high and low, you can have variation. However, if I am right in that your problem is that it's getting too cold in your room, the controller will just remain in the ON position and the tank temperature will continue to drop.
Just to confirm my guess, what's your room temperature at night and what is it during the day?