The easiest way to cool a fish tank is by using a commercial water chiller. However, chillers are often expensive, so aquarists have devised a number of techniques for lowering the temperature of their tanks. Because evaporation produces a cooling effect, most alternative methods for cooling an aquarium rely on it to some extent. For example, blowing a fan across the surface increases the evaporation rate and cools the water.
Some aquarists blow a fan across the water at all times, while others rely on timers to shut the fan off periodically or thermostats that turn the fan off once the water temperature drops. All three strategies have pros and cons and vary in their effectiveness depending upon the details of the aquarium and the room in which it resides. Some hobbyists attach a wet towel against the glass of the aquarium. As the water in the towel evaporates, the temperature of the aquarium glass drops, which chills the water.
Some hobbyists make their own chillers, which is often cheaper than purchasing a commercial unit, but requires considerable time and effort. Another simple technique is to freeze a small amount of aquarium-safe water. Once it is frozen, place the ice inside the aquarium. As the ice melts, it slowly lowers the water temperature.