Why Your Fish Tank Heater Light Is Not On & What You Need To Do About It.

In this article, you’ll learn what the flashing lights on your aquarium heater do and what they mean.

Most good quality fish tank heaters will have some indicator light that tells you whether the heater is on and working correctly,

But sometimes it’s off!

If you notice that the light on your heater is off or flashing, you might be wondering;

  • What does this mean?
  • Is it normal for the light to turn off?
  • What should you do about it?

When the light on your fish tank heater is not on, it means that the heater is turned off. All heaters will turn off & on automatically to regulate the temperature of your aquarium and prevent overheating. 

Keep reading to learn everything you need to know about the lights on your fish tank heater.

Related: How To Use An Aquarium Heater

Top Heaters We Use


What Does The Light On Your Fish Tank Heater Do?

The light on your aquarium heater is an indicator light.

The light will turn on when the heater is actively working to heat the water, and it will turn off whenever the heater not on.

In most cases, the heater and the indicator light will turn off and on to keep the tank’s temperature from climbing too high.

Note: Most heaters will allow you to adjust the temperature manually, and the heater will adjust accordingly until the set temperature is reached.

Why Does The Light On My Fish Tank Heater Keep Turning Off And On?

Typically, the light on a fish tank heater keeps turning off and on because the heater itself is turning off and on as it heats the water until the desired temperature is reached.

However, if you notice that the indicator light is turning off and on rapidly, this is a signal that something might be wrong.

The problem is not the light, but the heater.

If the heater is attempting to cycle through being on and off too quickly, it is probably too large or too powerful for your tank.

Why Is My Fish Tank Heater Light On But Not Heating Up The Water?

Sometimes, the light may come on, but the heater and the water do not feel warm,

If your aquarium heater light comes on even though the heater is not heating the water, there is probably a loose or broken electrical contact inside the heater.

Whenever this happens, the best thing you can do is replace your fish tank heater, as there is no simple fix to this issue.

Note: Always use a thermometer to monitor the temperature in the water.

Why Is My Fish Tank Heater Light Flashing? What It Means!

Sometimes the indicator light might start flashing.

This can mean one of the following things depending on how it’s flashing.

However, it’s important to note that every heater brand uses its own colors, and the below might not be specific to your heater brand.

The best thing would be to consult your user manual.

Here are the most common reasons;

Light Is Flashing Red

A flashing red light is not a bad sign in most cases.

The red light means that your fish tank heater is in the process of reaching the set temperature, and it is heating the water.

Light Is Flashing Green

A flashing green light is also usually a good thing.

This means your aquarium heater has reached the temperature it’s been set to.

After flashing green for a while, it will probably turn off or go back to red if the water gets too chilly.

Light Is Flickering Or Blinking

If either color of light is flickering or blinking, the water temperature might be fluctuating too fast.

This could be because the heater is running through its cycle too quickly.

Or, there might be an issue with the power or electrical components of the heater.

Reasons Why Your Fish Tank Heater Won’t Stay On?

If your heater won’t stay on, it’s completely normal.

However, if you have a suspicion that it’s not working correctly, then test the water with a thermometer. If the temperature is correct, then it’s likely fine.

Below are a few reasons that your aquarium heater is not staying on;

They’re not all bad!

The Desired Temperature Has Been Reached

Your heater will not stay on always because it has already reached the ideal temperature for your fish tank, and it would overheat the water and the fish if it kept running.

Again, if you see the light flashing green before it shuts off, it has reached the set temperature.

Some heaters will display the temperature with a color so that you know if you’re within a safe range.

But it never hurts to confirm the temperature with a floating thermometer.

Your Heater Is Malfunctioning

The heater might be malfunctioning if you notice any of the following;

  • The heater does not seem to stay on for long periods.
  • The lights are flickering rapidly.
  • The water is cold.

Any of these reasons might be an indicator that the heater is malfunctioning.

Some heaters do not work well if they are fully submerged, so make sure you know which type of heater you have.

Is it submersible or only immersible? To learn the difference, read this: Can You Fully Submerge a Fish Tank Heater?

Sometimes adjusting the heater so that only the heating element is below the water might help.

However, if that does not work, you should replace your aquarium heater.

Problem with the Power Supply

If there is a problem with the amount of power getting through to your heater, it will likely turn off for extended periods.

Always ensure that you have plugged in the heater correctly and the outlet is powered.

You could try plugging the heater into a different outlet to see if that fixes this issue.

If the outlets aren’t the issue, it may be an internal contact issue, which calls for a replacement heater.

Do Aquarium Heaters Turn Off Automatically?

Yes, heaters are supposed to shut off when they reach the set temperature and turn back on when the water starts to get too cold.

This is not something to worry about unless the water temperature is not where you set it.

Otherwise, the heater would fry your fish by overheating the water.

Video: How To Check If Your Aquarium Heater Is Working

Key Takeaways

Your fish tank heater is an essential piece of equipment for your tank, and it needs to be working correctly.

Check the temperature of your water every day.

If you notice anything odd, use some of the above steps to check if your heater is working correctly.

Lastly, it is normal for your heater and the indicator light to turn off and on as it reaches the right temperature setting in your tank.

Tank on.


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