An under tank heater is only going to create a hot spot, not change the ambient temp, so if it's varying hugely you should find a area that stays a stable temp. Sunlight shining in a glass tank is not a good idea and can get dangerously hot very fast so I would move it or black out the side getting light to prevent that. I use herpstats for thermostats and would recommend them, along with accurate thermometers for a quick temp check and a temp gun for more accurate numbers.