Instructions
Protect the aquarium from direct sunlight. Because full aquariums are extremely heavy, this involves planning where to put the aquarium before you set it up. If you must place the aquarium near a window, figure out a way to block the sun's rays from hitting the tank.
Cool the air in the room where the aquarium is housed by closing curtains and blinds, turning on fans and running an air conditioner.
Replace incandescent aquarium bulbs with fluorescent bulbs. Incandescent bulbs produce a lot of heat.
Raise your aquarium lights so there is space for air to circulate between the light fixture and the water surface.
Remove the lid or hood from the top of the aquarium, exposing the water surface to the air. Cover the top of the aquarium with netting or a screen if you are worried your fish will jump out.
Place a small fan near the tank's edge so it can blow across the surface of the water. The air movement causes water evaporation that cools the tank. Monitor your water levels if you use this method.
Aerate the water using a bubble filter. In addition to promoting evaporation, the filter helps add oxygen to the water.
Freeze water in plastic bottles, seal the bottles, and then float them in your tank. Replace the bottles when the water inside them thaws or when your aquarium has reached the appropriate temperature.
How to Keep an Aquarium Cool in Summer Without a Chiller
Many aquarium keepers invest in expensive water chillers to bring down high water temperatures, but some do-it-yourself methods also help beat the heat. Aquarium keepers have to keep a close watch on the temperature of their tank's water. Strong aquarium lighting and high ambient temperatures all work to raise the aquarium temperature, which is a problem because warm water holds less oxygen than cooler water. Tropical freshwater tanks can be kept at temperatures ranging from 72 to 82 degrees Fahrenheit, depending on the type of fish you are raising. Saltwater tanks should be maintained in a more narrow temperature range, from 72 to 78 degrees Fahrenheit.