A couple of suggestions: Heaters are not always reliable and I have had problems with a bunch of them over the years maintaining stable temps. The VisiTherm has been the most reliable one for me.
What wattage is your heater? You don't need a large one for that size tank. I would suggest pulling it out of the tank in the morning, before the lights come on and when the tank is cool, and keeping it out of the water for about 30 seconds or so to see if it gets warm and is actually working for you. Also, do you have a cover on your tank? That temp variation is pretty extreme. Then there is the issue of your thermoters-can you calibrate the one you are using to see just how accurate it is?
My 12G nano stays pretty stable, even without a heater, and I have PC lighting on it.