.....Heaters have a bad habit of shorting out and remaining on. Being in business I hear this story about once a week. For the most part you don't need a heater at all. Pumps and lights keep a tank warm from their excess heat. It's OK to let your tank warm during the day and cool at night. The ocean does the same, especially in the shallow reef areas.
.....During low tide, the small pools left exposed often reach 100 degrees in the tropical sun. Small fish, inverts and corals survive just fine until the tide returns. I don't recommend that you let your tank warm to 100 but you can let it rise and fall 6 to 8 degrees daily somewhere in the range from 68 to 83. My shop tanks vary 6 or 7 degrees over 24 hours. This prevents thermal shock when there is a greater change in the tank from an especially hot or cold day. If you keep your tank at a fixed temperature during a very hot day and your tank suddenly jumps 10 degrees, the fish, corals. etc. really suffer.
.....If your situation is such that you REALLY do need a heater then put it on a timer. Set the timer to come on for a 4 hour period early in the morning; or in extremely cold situations for two 4 hour periods at different times of the day.
