Originally Posted by
Willie
I find it's less the size of the tank then the temperature difference between the room and the tank temperature. Jacob cites an extreme case with 600W total for a 75 gallon tank in a cold location. I have a 125 gal tank sitting in a walk-in closet adjacent to my bedroom. It has 1 X 100W and 1 X 200W and the larger heater (which I set 2 degrees warmer) only comes on during a big water change. For esthetic reasons, my heaters are on the same side of the tank occupying different walls, but I have very good flow in the tank. My recommendations are as follows:
1. Always use 2 smaller heaters vs 1 big one.
2. Set one 1 - 2 degrees warmer than the other one.
3. Use different wattage heaters for maximum flexibility.