How to Cool Down a Hot Aquarium

Freshwater fish don't like change. You need to make sure the temperature and chemical composition of their water stays as steady as possible. If their water gets too hot, your fish will die. More often, the lack of oxygen in the water will kill them before the heat does. (The hotter the water, the less oxygen in it.) But you need to cool off your tank bit by bit, so your fish have enough time to adjust to the falling temperature.

Things You'll Need

  • Aquarium thermometer (floating or stick-on)
  • Siphon hose
  • Bucket
  • Fan
  • Water conditioner (if necessary)
  • A second airstone and hose attached to your air pump (optional)
  • Hot water bottle (optional)
  • Aquarium chiller (when all else fails)
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Instructions

  1. Stay Cool, Fishies

    • 1

      Get in the habit of checking the temperature of your aquarium water at least once a day. Most freshwater fish can't tolerate temperatures over 86 degrees Fahrenheit or any sudden spike in temperature. Check for signs of heat stress in your fish. Fish that are too hot will seem in to be distress: They won't move around very much, and may gasp for air at the surface of the water.

    • 2

      Turn off the aquarium heater, if there is one in the tank. Unplug it and remove it. If the heater seems very hot, it might not be broken.

    • 3

      Determine the reason why the water is so hot. Is it the weather? Put up a shade over your fish tank, if possible. Even an open umbrella might help.

    • 4

      Move a fan toward your tank to blow cool air on it, if possible.

    • 5

      Turn up the power on your air pump, if possible, so that your fish get more oxygen in their water. If your air pump can't be adjusted, you need to attach an airstone to an air hose and attach that to your air pump, so that there are two airstones in your tank. Adjust the strength of the air flow or position the second stone so that the water currents don't knock your fish around.

    • 6

      Fill a hot water bottle with cool water (not ice water) and float it in your fish tank.

    • 7

      Do a partial water change. With your siphon, hose and bucket, remove 1/5 to 1/4 of the water in the tank. Dump the old water. Fill the bucket with lukewarm or cool (not cold) water. Add any dechlorinator or other water treatment you would add during a normal water change. Add the new water gradually; don't just dump it all in at once.