When the Aquarium Heater Light is On but Not Working?

If your aquarium heater light on but not working, there may be a problem with the heater. The fish will thus be at grave physical risk.

After lengthy usage, the heater’s components may get damaged. Additionally, you can receive a heater that has a manufacturing defect or was damaged in transit.

I’ll explain how to spot a broken heater, how to stop it from happening, and a lot more in this article. Continue reading to find out more about an illuminated aquarium heater that doesn’t function.

What are the Reasons for Aquarium Heater Light On But Not Working?

Even if the indicator implies that the aquarium heater is operating, there are several reasons why it may not be. Let’s investigate these reasons:

  • The heating pad of the aquarium has become faulty.
  • The heater has overheated because the water level has drastically dipped below it.
  • The heater’s internal parts have been damaged during shipment.
  • If the heater is submersible, turning it on immediately away after submerging it might cause harm.
  • The heater’s internal connections may be loose.
  • The light will continue to illuminate without producing any heat if the heater’s heating coil develops a problem.
  • The temperature has been set to a very high amount for a prolonged time.
  • If the heater was purchased from a retailer at a low price, it can contain manufacturing flaws.
  • A sudden change in electricity flow has damaged the internal parts of the heater.
  • The aquarium is too big compared to the heater’s output power.

These are the most typical reasons why the aquarium heater’s light is the sole component that is operational.

How to Ensure That the Aquarium Heater is Working Properly?

You should check a few things if you suspect that the heater is not heating the water. Examine the following items in great detail:

  1. You should check the water’s temperature at the beginning. Utilize a thermometer by submerging it in the water. The heater is not operating if the water temperature falls below the predetermined level.
  2. If you find that the water’s temperature is greater than the designated limit, you only need to raise it to the fish’s preferred temperature.
  3. Examine the fuse. The fuse may have blown if the heater is not functioning. The fuse needs to be changed.
  4. The heater will take longer to increase the temperature if you’ve just changed the water. Just give it a little more time.
  5. Check to see if the aquarium heater is strong enough. For every liter of water in the aquarium, one watt of power is required. You will require two watts per liter if you reside in a colder environment.
  6. Verify that the submersible heater is completely submerged. Installing a hood to keep the water level higher than the heater may be necessary if the water evaporates fast.
  7. Check the heater’s internal components and connections to make sure everything is in working order.
  8. You should change the heating pad and inspect the temperature.

How to Choose Heater Power According to Aquarium Size?

You can’t just use any heater with your aquarium. You need to meet the power requirement of your heater according to the amount of water inside the aquarium.

The safe choice is to equal the power of the heater with the aquarium’s volume in liters. So, if the aquarium can contain 50 liters of water, you need at least a 50-watt heater.

Take a look at the aquarium heater power guide in comparison to the volume of the aquarium.

Aquarium Heater Power Chart

Aquarium Volume Power for 5 Degree Celsius Increase Power for 10 Degree Celsius Increase Power for 15 Degree Celsius Increase
20 Liter 25 watt 50 watt 75 watt
40 Liter 50 watt 75 watt 75 watt
75 Liter 50 watt 75 watt 150 watt
100 Liter 75 watt 100 watt 200 watt
150 Liter 100 watt 150 watt 300 watt
200 Liter 150 watt 200 watt Two 200 watt
250 Liter 200 watt 250 watt Two 250 watt
300 Liter 250 watt 300 watt Two 300 watt

In the above chart, the average room temperature is considered to be 20 degrees Celsius. So, if you need a 20-liter aquarium’s temperature to increase by 10 degrees Celsius, you need a 50-watt water heater.

How to Replace the Aquarium Heater Heating Pad?

A broken heating pad is one of the primary causes of aquarium heaters that don’t produce heat even when the light is on. I’m going to provide you with the directions for changing the heater’s heating pad in this section.

Follow the instructions carefully:

  1. Unplug the heater and let it rest for 10 minutes. Remove it from the water and clean it with a toothbrush.
  2. Separate the heater’s dial once it has been dried. Using the opening tools or spudgers, disconnect the rubber stopper.
  3. Now carefully detach the glass tube from the primary components. Then, remove the foam insert from the heating pad and put it aside.
  4. Put on protective gear including gloves and safety glasses as you disconnect the power connections.
  5. Separate the chip from the heating pad. Detach the pins by heating them. Now, insert the new heating pins and solder the pins and the board.
  6. After that, secure the circuit board chip and place the heating pad in the proper spot. Now, place the foam inside the heating pad.
  7. Connect the glass tube to the heating core. Now, insert the rubber stopper by applying pressure.
  8. Finally, connect the plastic cap and align the switch properly.
  9. The heater should function flawlessly if all of these instructions are followed properly. For 10 minutes, submerge it in water. After that, switch it on and check the temperature after a few hours.

Which Tools Do You Need to Replace the Aquarium Heater Pad?

If you wish to repair the heater pad of your aquarium heater, you might need to purchase a few items. Look at my recommendations below:

  1. Power Probe Butane Soldering Kit
    – Includes 4 piece tip sets, PPMT tip, heat shrink, and 5 packs of solder.
    – Useful for soldering with an adjustable flame length of 0.5 to 1.25 inches.
  2. Kester Wire Solder, 0.031″
    – Made of 60 percent tin and 40 percent lead.
    – 1 pound of instant action solder with a diameter of 0.031 inches.
  3. Klein Tools Electricians Gloves Large
    – Made of 50 per cent TPB, 30 per cent leather, and 20 percent neoprene.
    – Safe for electrical usage with slip resistance property.
  4. Menda Nylon Spudger Repair Tool
    – Has 2 ends: Pry tip type and notched type.
    – Nylon finish which is safe for delicate objects.
  5. Weller Soldering Work Station Mat
    – Made of silicone with a dimension of 21.5 x 12.75 inches.
    – Has many sections and compartments for placing tools.


A defective heating pad is the major cause of your aquarium heater’s inability to heat the water, even while the indicator light is on. The heating pad replacement usually solves the issue.

Other problems may also contribute to this one. I’ve covered all the causes and solutions to the problem – the aquarium heater light on but not working. Follow my instructions thoroughly to fix the temperature issue with the aquarium heater.


Arif H Fahim

Welcome to PetFishTank! My name is Arif. I make an effort to publish educational articles using my many years of expertise as a fish owner.

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