It makes perfect sense, his lights are causing his temp to fluctuate. I had the same problem, before getting my lights my heater was on constantly and the temp was always at 78 degrees. once i got new lights my temp was 83 degrees when they were on and 78 when they were off. So you have to buy a chiller, or a fan to cool the water off. Keep the heater at the same setting, and my advise would be to keep the fan on at all times. Although you will deal with more water evaporation with this, you won't have those swings of "oh crap is noon and my temp is 82 degrees" you turn on the fan and it dips back down to 78. It needs to be constant, so keeping a fan on at all times will keep the water cooled at 78 during the day, and at night your heater will ensure the water stays at 78 and doesn't drop below.
Some people sync their fans with their lighting schedule - it just depends on your situation. I would place one fan on your sump at first (so that it isn't an eye-sore), then if that doesn't help either buy more fans, or place anotehr one on your DT.
hope this helps.