A constant, appropriate water temperature is vital to keeping your aquarium's inhabitants healthy. A sudden temperature change can place undue stress on your aquatic life, increasing their susceptibility to infection and disease.
Aquarium setups - reef aquariums in particular - often employ equipment, lighting and water pumps especially, that produce enough heat to raise water temperature. Without a chiller to decrease water temperature, aquarium water can warm to a point where it no longer holds enough oxygen to sustain the inhabitants.
If you need a chiller, plan it into your system as early as possible. Chillers are external units that often require plumbing and always require adequate space and sufficient ventilation. They give off significant amounts of heat when in use. In fact, you should avoid placing your chiller in an enclosed aquarium stand, because it can give off enough heat to raise water temperature.
Factors to consider when choosing a chiller
When choosing a chiller, consider your aquarium size, additional equipment and the heat it generates, your aquarium setup, and the temperature requirements of your aquarium inhabitants. Choose a chiller that effectively cools your aquarium water in the space and setup you have allotted. If your cooling requirements are on the upper end of what is recommended, choose the next largest chiller to ensure an adequately cooled aquarium and healthy inhabitants.