If you have good circulation the temperature should be nearly the same every where in your tank and sump except the very thin skin of water at the surface of your tank and sump top surface which will be a very little bit cooler due to the evaporation cooling effects. With adequate tank circulation the temperature of your sump water will match your tank water. All parameters for all parts of a tanks system should always basically be the same. There will be exceopptions to the rule like a higher dissolved oxygen levl near the exit of a skimmer or in a refugium with macroalgae when the fuge light is on, but with good circulation even these should be diificult to detect. It would be easier to make good suggestions about your heating problems if more dat was available. Fot instance what is your normal room temperature. How much water are you loosing to evaporation every day. With 72 gallon tank it would be nice to shoot for 2 gallons per day of evaporation with fans running only while the lights are on , as you wabnt the evaporation to happen only when you want to drop the tanks temperature. Water evaporation removes huge amounts of heat energy from water bodies (IE your tank). Fans increase the evaporation by moving the moisture laden air out of the way and bringing in replacement air that is drier so that it can pick up moisture to carry away. If you live in a drier climate it works better than if you live in a humid climate. It works very well in the dry southwestern part of the United States. The fans basically turn your sump into a "swamp cooler."