Keeping a constant temperature is very important to all fish and corals in a saltwater or freshwater aquarium.
Many of the steel and titanium aquarium heater controllers on the market have a 3 degree temperature variance either way. That means your fish tanks water can vary actually up to 6 degrees.
If you want healthy fish then that is not something you want to mess around with. The Truetemp Digital Heater Controller is what you want for precise control over your heater. This unit will work with Won Brothers heaters and most other units that require a controller.