While most people would purchase a heater suited to the size of their aquarium, others choose to purchase larger heaters, reasoning that the heater will last longer. While a larger heater might last longer due to the relative amount of work the heater has to do (a larger heater will maintain the temperature with less work), purchasing a larger heater is NOT a wise choice. The reason for this is simple; if the heater malfunctions so that it is permanently on, then the fish in your aquarium will be effectively boiled alive. Purchasing a heater rated to the aquarium in consideration will give you an opportunity to recognise a problem in the event of a malfunction. In other words, it will take an appropriate sized heater longer to ‘boil’ your fish than a larger one.