Why No Heaters .....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 1 to 4 hour period early (depending on the size of your tank) in the morning; or in extremely cold situations for two 1 to 4 hour periods at different times of the day. At present I only sell "Stealth" brand heaters. They claim to shut off if they fail.
I said several small, but that’s not the whole story.
I do run several small heaters, but the total wattage is only about half of what is recommended for my tank size. My 180 gal has 3 heaters rated in total for about 100 gal, I see 2-3 degree swings during the day.
I have 1 small heater that is on both a temperature regulation setting, and it can only turn on at night. Mind you, I have a 30gal with a 20gal sump, and a 10gal refugium. So . . . total volume of 52-55gal?
With such a small volume, my lights keep the temp up during the day and it only needs regulation at night. In the winter I will add another small heater in the sump that will be set to only turn on during the day.
I only use a heater when it starts getting cold. I have not seen my tank go below 75/76 and no higher then 80 since the warmer months came. Before I was using a heater in my overflow with a temp probe and 1 in the fuge area for about 300watts worth on a 75g. This year I would like to get another heater with the temp probe and use one maybe in tank and other in fuge. I got the over flow idea from Moliken’s tank. It worked well.
Yup, i use heater to support the bottom end. usually about 77 - 78 degree setting. usually just one 100w heater for a 80-100 gallon system. during the summeeri remove them. the tanks normally warm to 82-83 by the end of the lights on cycle, then cool to 77 by morning. it is a perfect reef cycle. the house is heated/air conditioned so room temp is arount mid 70s . it cant go too far wrong. I think it makes the corals and fish more adaptable and able to cope with changes.
there was an actual reef study by the university of Hawaii some years ago. they placed dozens of instruments all over their reef. from deep outer slope, mid slope, reef crest, lagoon side and tide pool zones of the reef . they monitored things like PH, temp, alk, over a year time frame. so they not only have daily cycles, but seasonal, and anomalies like storms. i’ll post it if i can find it again. it showed wide swings on temp per day and seasons.