There will be a difference in room temperature vs tank temperature based on a few things. One is the fluctuations in the room temperature. That will depend on your heating/cooling settings for day/night times. If you don't control control your temps that way, then the natural day/night temps will probably affect them more. Water is much more dense than air is, so just like the oceans/lakes there is a delayed effect on heating/cooling a tank vs the ambient air temps. The larger the volume of the tank the longer it will take for the tank to reach ambient temperature. Also the type/quantity of accessories, especially the lights, will add heat to the tank. This heat will add or subtract to the delay depending on if you are in a heating or cooling cycle at any given time. It is possible that even once the tank has had time to reach ambient temps, it could surpass ambient if there is enough heat being added from the lights/accessories.