How to Lower High Water Temperature in Aquariums

By
Shirlie Sharpe
Shirlie Sharpe
Shirlie Sharpe is an aquatic expert and writer with over three decades of experience keeping and raising ornamental fish. She has consulted with The Minnesota Zoo and the National Aquarium. Shirlie has also authored a book on setting up aquariums.
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Updated on 12/11/25
Reviewed by
Nick Saint-Erne
Nick Saint-Erne, DVM
Reviewed by Nick Saint-Erne
Dr. Nick Saint-Erne, DVM, is a highly accomplished veterinarian and writer who has treated zoo animals and exotic pets for more than 35 years. He has worked with the Centers for Disease Control and Prevention to improve pet store animal care among other endeavors. Dr. Saint-Erne is part of The Spruce Pets' veterinary review board.
Learn more about The Spruce Pets' Veterinary Review Board
Aquarium with Goldfish
Credit:

Rebeca Mello / Getty Images

Key Takeaways

  • Monitor aquarium water temperature during heatwaves, as high temperatures can harm fish and reduce oxygen levels.
  • Use methods like increased aeration, ice packs, and reduced lighting to cool aquarium water gradually and safely.
  • Prevent overheating by keeping tanks out of direct sunlight and performing regular water changes with slightly cooler water.

Most people know they need to provide a cool area for their cats and dogs during summer heatwaves, but what about fish? Just because your fish are in water doesn't mean they're safe from the effects of high temperatures. in the event of a power failure, or lack of air conditioning, during the summer, the aquarium water may heat up higher than the safe limits for the fish. It is important to know how to cool the aquarium water in these unusual circumstances. 

Safe Temperature Range

Heat can become a problem if it is markedly elevated for a considerable period of time, which might be possible during a prolonged hot spell. The Angelfish, Guppies, Mollies, and Silver Shark, will have absolutely no problems with a water temperature of 81 degrees Fahrenheit, or 27 degrees Celsius. The Clown Loach enjoys water as warm as 86 Fahrenheit and 30 degrees Celsius and is probably thanking the weather person for the lovely warm water.

As long as the water temperature does not remain above 86 degrees Fahrenheit and 30 degrees Celsius for weeks on end, you need not be concerned. However, as the temperature rises, the dissolved oxygen in the water decreases. If you have a good filtration system, odds are you won't have a problem. Nevertheless, it won't hurt to increase the aeration to ensure proper levels of oxygen. You should also perform water changes more often, using water that is a degree or two cooler than the tank water. That will serve to keep the water temperature down and maintain adequate oxygen levels for your fish.

Illustration depicting the ways to cool aquarium temperatures
Credit: The Spruce / Sunny Eckerle

Reducing Water Temperature

In the event of a lengthy heatwave, you might have to take steps to cool the water. Here are some suggestions for keeping the water cool and ways to lower the temperature if it goes too high. 

  • Keep the aquarium lights turned off.
  • Make sure the room does not receive direct sunlight.
  • Remove the hood from the tank (use caution if your fish are jumpers). Also, keep an eye on any cats you may have. 
  • Place a fan so that it blows directly across the water.
  • Float ice packs in the water.

Regardless of which methods you use, be sure to keep the rate of reduction slow, dropping about 2 degrees Fahrenheit or 1 degree Celsius every eight to ten hours. Remember, rapid water changes are harmful to your fish. You want to keep an eye on your aquarium's water temperature from the time the heatwave starts. You want to do your best to keep your aquarium cool as the room temperature rises, rather than have to find a way to cool it off. 

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