I have an aqueon 100 watt heater that's preset to 78 degrees. it's in a 20 gal tank, right by the filter output, and it's been there for 2 weeks. yet.. it has failed to keep the tank 78 degrees, like it should. the temperature keeps fluctuating, 75-77 degrees. this morning the temperature read 75 on both my digital thermometer and the little thermometer stuck to the side of the tank. it was somewhat understandable, considering I had the fan on in my room. for future reference, should I be alarmed about temperature fluctuations hurting my fish, and what should I do about this? maybe add a small heater to the other side? leave it as it is? try to return the current heater and get a new one?