Aquarium Heater Adjustment

Jeager, Eheim 100W Aquarium HeaterCreative Commons License courtesy of Kemal Y.

For safety and convenience, buy a completely submersible aquarium heater, and read the label just to be sure. The temperature dials on most aquarium heaters are far from accurate. Thus, setting the temperature is a step by step process requiring the help of a good thermometer.

  1. First, set the thermostat at a slightly lower temperature so the heating element turns off.
  2. Wait several hours for water temperature to stabilize, then measure it.
  3. Always measure temperature at the same water depth.
  4. When the temperature stabilizes to a couple degrees below the desired level, adjust the thermostat so the heating element just turns on.
  5. Wait again for the temperature to stabilize, then re-measure it and make small, incremental adjustments as required.

If the aquarium is large, it will take quite some time for the temperature stabilize. Since water acts as a heat sink, it stores and releases energy very slowly. A large volume of water heats and cools much more slowly than a smaller volume.

The heater should keep aquarium temperature to within a few degrees of the setting. This avoids sudden and stressful temperature changes. Even if the aquarium room occasionally gets warm, a thermostatically controlled heater will help stabilize aquarium temperature at least at the low end. Different species of aquarium fish need different temperatures, so factor that into fish selections and thermostat settings.

For additional technical details, please click the following link:

So... what do you think? Please leave me a comment.


  • Vale: Are there any aquarium heaters that are reliable?
  • johnarthur: Not in the long run. Any heater can fail, and if it fails in the ON mode the fish get very hot.
  • Vale: There’s no emergency automatic shut off if it exceeds a certain temperature?
  • Zach: Great blog!
    Would you be able to do one for changing aquarium heaters?
  • Zach: Like changing a heater for a tank that already has fish in it?
  • debcc: Nice blog! I was all freaked out at first because the temperature in the water didn’t match what I had it set at. It did take some playing around but I finally got it right :)
  • Cynthia: Great Blog A.J!
    Yes heaters are unreliable unfortunately. However, I live in a very cold state. We get snow all the time and only have a few months of summer(like 3?)so a heater for me is a must. So far none have exploded lol
  • johnarthur: Some aquarists put two heaters in the tank just incase one fails. That also distributes heat more evenly than only one heater. Of course, you will still need to monitor temperature to catch any runaway heating.

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