Heating a tropical aquarium in the winter is relatively easy with heaters, but keeping it cool enough during hot weather can be a challenge without a chiller. Because chillers are expensive to buy and run, many aquarists instead choose to rely on other methods for maintaining appropriate temperature.
One of the most common aquarium cooling methods is the use of fans. Small fans can blow across the surface of the aquarium, the sump or both. Manual fans are suitable for use on particularly warm days; more intricate ones are automatic; timed to operate on their own at pre-determined points. Meanwhile, you can position fans to exhaust hot air created by the lights and filtration equipment and to draw in cooler air from outside. When using fans, it's important to top off evaporated water. If the tank is a saltwater setup, it is important to maintain a consistent salinity, topping off with freshwater.
Another solution for keeping an aquarium at the right temperature during hot weather is a window-based air-conditioning unit. Rather than controlling just the water temperature with fans, an air-conditioning unit will cool the entire room and help maintain an appropriate environment for your aquarium inhabitants. In addition to creating a more comfortable space in which to observe your aquarium in the heat, cooling the entire room is beneficial if you use various pieces of equipment that get warm. How well the room is insulated and the cost of energy in your region will help you determine whether an air-conditioning unit is the right choice for you.
In the event of an emergency, several methods of last resort can cool an aquarium without a chiller. The most common is the use of ice or cold packs. Place the ice or cold packs in a clean, strong plastic bag, and place the bag directly in the sump. If you don't have a sump, you can place the bag in the aquarium itself. It takes a lot of effort and ice to cool the aquarium in this manner, but it's effective in an emergency. Another emergency method, which may be used in combination with ice is to turn off all nonessential equipment. Pumps and lights, especially metal halides, add heat to the system. Turning some of these off can help in an emergency. Be sure, however, to maintain adequate flow to properly oxygenate the water. Also, be sure to clean and restart any biological filters that may have been impacted by turning off various pumps.
While a chiller may be the best choice for some tropical aquaria, employing a combination of alternative cooling methods is effective for many aquarists. Plan for year-round temperatures and especially for unexpected hot-weather events. Using an aquarium controller will ensure that your aquarium is ready even when you are not. Set up a multi-tiered system, whereby your aquarium controller monitors your water temperature and automatically addresses problems by first turning on fans and then turning off lights and other nonessential equipment if the temperature continues to rise. You can set top-line aquarium controllers to email or text you an alert so you can take more drastic measures if necessary.
Ret Talbot is an award-winning writer and photographer, as well as a lifelong freshwater and marine aquarist. He is the author of "Banggai Cardinalfish; A Guide to Captive Care, Breeding & Natural History" and "The Complete Idiot's Guide to Saltwater Aquariums." He is also a senior editor at "CORAL Magazine," the world's leading reef and marine aquarium magazine.