REDDIT – Why Your Fish Tank Heater Light Is Not On & What To Do

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

Table of Contents

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

A. Importance of Maintaining Proper Temperature in Aquariums

Maintaining the correct temperature in your aquarium is crucial for the health and well-being of your aquatic pets, especially tropical fish. These beautiful creatures are highly sensitive to temperature changes, and even a slight deviation from their ideal environment can lead to stress and illness.

B. Common Issue: Fish Tank Heater Light Not On

One common problem that aquarium owners encounter is when the heater light on their submersible heater does not turn on as expected. This issue can be quite alarming, as it raises concerns about the temperature of the water in the tank.

C. Overview of Keywords

In this article, we’ll delve into the significance of temperature control in aquariums, explore the reasons behind the heater light staying off, and discuss various aspects of aquarium heaters and their maintenance. We’ll also provide insights into different types of heaters and offer tips to help you ensure the well-being of your aquatic pets.

Understanding the Significance of Temperature in Aquariums

A. The Impact of Temperature on Tropical Fish

Tropical fish thrive in specific temperature ranges, which mimic their natural habitats. Variations in temperature can affect their metabolism, immunity, and overall health. Maintaining a consistent temperature is vital for their well-being.

B. The Role of Submersible Heaters in Maintaining Ideal Temperature

Submersible heaters, such as Eheim Jager heaters, play a critical role in aquariums by regulating water temperature. They ensure that the tank remains within the desired temperature range, creating a comfortable environment for your aquatic pets.

C. Importance of Room Temperature in Aquarium Maintenance

The ambient room temperature where your aquarium is located can impact the effectiveness of your heater. Understanding how room temperature affects your aquarium is essential for successful temperature control.

Diagnosing the Problem: Heater Light Not On

A. Checking for a Faulty Heater

1. Evaluating the New Heater

When you encounter the issue of the heater light not turning on, one of the first things to consider is the condition of your new heater. It should function correctly and maintain the desired temperature range.

2. Assessing the Condition of Eheim Jager Heaters

Eheim Jager heaters are renowned for their reliability, but even the best heaters can experience issues over time. Inspect your Eheim Jager heater to ensure it’s in good working condition.

B. Analyzing the Cold Water Situation

1. The Temperature of the Water

Before troubleshooting further, measure the temperature of the water in your tank. Ensure it matches your set temperature on the heater.

2. Heater Light Stays Off due to Broken Heater

A common cause of the heater light staying off is a broken heater. Check for visible damage or irregularities in the heater’s glass tube or heating element.

C. Examining Heater Components

1. LCD Screen and Temperature Dial

Inspect the LCD screen and temperature dial on your heater for any inaccuracies or malfunctioning components.

2. Heater Size and Wattage

Ensure that you’ve selected the appropriate heater size and wattage for your tank’s volume to maintain a consistent temperature.

3. Built-In Heater Guard and Glass Shield

Some heaters come with a built-in heater guard and glass shield to protect them from damage. Make sure these components are intact and functioning correctly.

Troubleshooting Heater Light Issues

A. Potential Causes of Heater Light Not Turning On

1. Faulty Sockets and High Resistance

Inspect the power socket and connections for signs of damage or high resistance, which can impede electrical flow to the heater.

2. Incorrect Temperature Settings

Verify that your temperature settings are accurate and within the desired range. Incorrect settings can lead to the heater light staying off.

3. Temperature Range and Preset Temperatures

Understand the temperature range of your heater and how to set preset temperatures to maintain a consistent environment.

B. Addressing Common Problems

1. Heater Part Failures

Components like nichrome heating coils may fail over time. If you suspect this, consider replacing the faulty parts.

2. Inaccurate Temperature Readings

Inaccurate temperature readings can result from sensor issues. Calibrate or replace the sensor to ensure precise monitoring.

3. Large Temperature Swings

Heaters should maintain a stable temperature. Address any large temperature swings promptly to prevent stress to your aquarium inhabitants.

Maintenance Tips for Aquarium Heaters

A. Regular Cleaning Schedule

Establish a regular cleaning schedule to prevent issues like salt creep and maintain a clean, functional heater.

B. Checking for Salt Creep and Affected Areas

Inspect for salt creep on your heater and nearby components. Clean affected areas to ensure proper functioning.

C. Ensuring Proper Water Level and Suction Cups

Maintain the appropriate water level in your tank and ensure the heater’s suction cups securely hold it in place.

D. Monitoring the Power Switch and Multi-Way Socket

Regularly check the power switch and multi-way socket for any signs of wear or damage, as these can impact heater performance.

Exploring Different Types of Aquarium Heaters

A. Eheim Heaters vs. Top Fin Heaters

Compare the features and performance of Eheim heaters with those of Top Fin heaters to determine the best fit for your aquarium.

B. Aqueon Heaters vs. Fluval M Heaters

Evaluate the pros and cons of Aqueon heaters and Fluval M heaters to make an informed choice.

C. Nichrome Heating Coils vs. Metal Body Heaters

Understand the differences between heaters with nichrome heating coils and those with a metal body, and consider which suits your needs.

Finding the Best Solution for Your Aquarium

A. Top Tips for Aquarium Owners

1. Consistent Temperature Maintenance

Prioritize consistent temperature maintenance for the well-being of your aquatic pets.

2. Avoiding Bad Heaters

Research and invest in reliable brands to avoid bad heaters that may cause problems.

3. Choosing the Right Heater Size

Select a heater size appropriate for your tank to ensure efficient temperature control.

B. Considerations for Larger Aquariums

Understand the unique requirements of larger aquariums and choose the right heater accordingly.

C. Extreme Cases: Dealing with Heater Problems

Explore solutions for extreme cases of heater malfunction, such as sudden temperature spikes or drops.

D. The Role of Built-In Thermostats and Air Stones

Learn how built-in thermostats and air stones can enhance temperature control and water circulation in your aquarium.


A. Importance of Resolving Heater Light Issues

Resolving heater light issues promptly is essential for maintaining a stable and comfortable environment for your aquatic pets.

B. Achieving the Correct Temperature for Your Fish Tank

Ensuring the correct temperature in your

fish tank is a key factor in the health and happiness of your aquarium inhabitants.

C. Ensuring a Healthy Aquarium Life Thermal Submersible Aquarium Heater

Consider investing in high-quality heaters like the Life Thermal Submersible Aquarium Heater for optimal aquarium health.

D. Selecting the Best Heater for Your Aquarium

Choose the best heater that suits your tank’s needs, taking into account its size, type, and the specific requirements of your aquatic pets.

E. Maintaining a Consistent Temperature for Happy Aquarium Fish

Maintaining a consistent temperature in your aquarium is a responsibility every aquarium owner should prioritize to ensure the well-being and happiness of their fish.

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.


Jack Dempsey
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