I don't use heaters in my 90 gallon tank, and it naturally stays between 80 and 84, no matter what time of day. I've noticed that my return pump and the pump that feeds my recirc skimmer put out a lot of heat, enough to keep the entire system warm. My lights (T5s) don't contribute any heat to the system.
I have one little heater for emergencies in my 10 gallon nano, and that kicks on at night. My 10 gallon tank usually stays between 80 and 82 and is heated by the ambient room temperature during the day time.
In my old 240, my lights ran so hot that it would heat up to 95 degrees during the day. I found that out the hard way -- when everything in my tank boiled and died right after I added the new lights! I added some clip on fans to it, and it dropped the temps down by about 10 degrees.