Kinda obvious question, but do you have a heater in the tank?
and btw...i know its kinda a obvius question for you guys...but note:im only 13...so its not really obvius to me...><
He was saying his question was obvious, not yours....
My first guess is that the lights are heating up the tank. Lights strong enough for a planted tank will warm up the tank. If the exact same thermometer is reading higher in the tank than in the room, then the tank is warmer than the room.
If the new thermometer is reading higher in the tank than what a different thermometer is reading in the room, you may need to calibrate your thermometer.
Can the thermometer read 0 F? If so, mix up a cup of ice water with a tablespoon of salt, let it set for 5 minutes, and stick the thermometer in. See what it reads and write it down.
Can it read 212F? If so, boil some water, stick the thermometer in and see what it reads. Write it down.
Make a graph in excel where the y axis is the real temperature (0F and 212F) and the x-axis is what your new thermometer read. Now it's calibrated.