If your room temp is 74 there is no way for your cage to get down to 68 wether a fan is on or not. I really think your Accurite is not working correctly. It is not possible to have over a 20 degree temps difference in a 15qt tub. Are you using the lid that came with the tub or a screen lid? Something just does not make sense here especially with you using 11' flex watt. I did have a problem with my accurite thermometers when the batteries were running low one time. Mine were incorrectly reading way to high but you might want to try changing out the batteries or even getting a new thermometer.