I would definitly follow the others' advice and think about replacing the heater since most references say there should be only about 2 degrees of fluctuation in you tank temp (between lights on and off). However, reef lighting adding too much waste heat or big fluctuations in the room tempature can cause similar problems. Have you changed anything in the tank (or room) lately? Is this a day-night fluctuation or a day-to-day fluctuation?
If you do decide to replace the heater, the usual rule of thumb is to have about 3-5 watts of heater per gallon of tank water. Of course, you have to take into account where the tank is located. If the tank is in a cold/ hot room, subject to drafts, etc. you may need to increase or decrease your wattage. Dividing this total wattage across two heater allows you the safeguard of having at least some heating in the tank should a heater fail. Additionally, having two heaters allow you to space them out in your tank/ sump for more even heating.
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