From what I have been told and thru reading...CF
bulbs should be replaced every 6 months (regardless if they still work or not). I use that schedule just to be on the safe side. Other may disagree.
As for heaters..the temp is going to fluctuate as the day goes on. Your lights will raise the temp, then after lights out the temp will drop to whatever your heaters are set at. Maybe you don't have enough watts per gallon of heating. My 100W stealth says up to 30 gallons. But I added another 50W just to be sure.