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.
An Insurance Policy…
However, I would take it one step further. If you have a 300 litre aquarium, I would purchase two 150 watt heaters or even three 100 watt heaters, instead of purchasing a single 300 watt heater. This will further prevent the chance of a heater malfunction causing mass fatalities within the aquarium.
As the value of the aquarium livestock increases it becomes even more imperative that you place more than one heater in each aquarium. While many may grown at the thought of the additional financial cost of purchasing more than one heater per aquarium, do not be discouraged. Purchasing two or more heaters per aquarium does not mean that you will spend more money in the long run. The reason is simple; the key is to have one heater set to the desired temperature of the aquarium and the others to a temperature two degrees lower. In the event that the main heater stops working, the others will kick-in and save your precious livestock. By rotating through the heaters, as each one fails or is replaced, you will still get the combined lifespan of all the heaters, meaning that you will spend the same amount of money in the long run.