Heaters are an essential part of any aquarium; freshwater or marine. However, most have a limited lifespan, with most brands having a guarantee of less than three years. (See: Aqua One Heaters) This limited guarantee on the longevity of heaters is partly due to the great degree of variability in the amount of time a heater will remain functional, with differences in set temperature and the heater-to-tank size ratio being the biggest contributing factors to this variability. While a simple solution to this problem would be to replace the heater every three years, regardless of its working condition, there is still the chance that the heater will fail before the three years are finished. The failure of a heater could have devastating consequences for the inhabitants of the aquarium. So, how do you prevent mass die offs? Simply purchase two heaters with every aquarium. This DOES NOT mean that you will go through twice as many heaters. The key is to set the heaters at different temperatures, with a two-degree difference being optimal. That way, one heater will heat the aquarium and the other will only switch on if the water temperature gets too low or the first heater fails. In practice, you will go through the same amount of heaters while eliminating the possibility of mass die off due to heater failure.