Miscellaneous

Aquarium heater isn't heating the water

A heater that doesn't raise tank temperature usually has a broken element, a miscalibrated dial, or simply needs time, and checking the actual water temperature is the key first step.

Difficulty
Easy
Time
Several hours to confirm (minimal hands-on time)
DIY cost
$15–$40
Pro cost
n/a

Symptoms

  • Water temperature stays below the set point for hours
  • Heater's indicator light doesn't turn on
  • Temperature swings unusually high or low
  • Heater feels hot to the touch but tank stays cold

Likely causes

  • CommonHeater not fully submerged or positioned correctlyMost heaters need to be fully underwater and have good water flow around them to work and to read temperature accurately.
  • CommonThermostat dial needs recalibrating or was bumpedAnalog dials can drift from the true temperature over time or be accidentally turned.
  • PossibleHeater wattage too low for the tank sizeAn undersized heater may run constantly without ever reaching the set temperature, especially in a cold room.
  • PossibleBurned-out heating elementAfter a year or more of use, the internal heating element can fail even though the unit still powers on.
  • PossibleThermometer reading is inaccurate, not the heaterA separate stick-on or digital thermometer can be wrong, making a working heater look like it's failing.

What to do

  1. Verify actual tank temperature with a separate, reliable thermometer rather than relying on the heater's built-in reading alone.
  2. Confirm the heater is fully submerged per its markings and positioned near a filter outlet or powerhead for water circulation.
  3. Check that the heater's indicator light behaves as expected (on when heating, off when at temperature, per the model).
  4. Let the heater run for several hours after any adjustment before judging results, since large tanks change temperature slowly.
  5. Compare your tank size to the heater's rated wattage; a general guide is about 3–5 watts per gallon, more in a cold room.
  6. If the dial has degree markings, do a small test adjustment and monitor temperature change over a few hours to check accuracy.
  7. Replace the heater if it shows no temperature change over many hours despite being submerged, powered, and sized correctly.

Tools and parts

  • Separate aquarium thermometer
Aquarium heaterSearch: “aquarium heater sized for your tank gallons”

When to call a pro

Aquarium heaters are inexpensive enough that replacement is standard practice once a failure is confirmed with a separate thermometer.

Safety

Use caution

Always unplug an aquarium heater before removing it from the water, since running it dry, even briefly, can crack the glass or cause a shock hazard.

Related guides

Operator provides AI-generated troubleshooting information. It can be wrong. For safety-critical, electrical, gas, structural, medical, automotive safety, or other high-risk problems, consult a qualified professional.