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Fish Tank Heater Light Blinking and Shutting Off

Preset aquarium heaters have an electronic thermostat that automatically turns the device on and off to maintain a constant water temperature (usually 26° Celsius).

An LED indicator lamp lights up to let you know the heater is running. But, what should you do if the heater light is flickering or it stops working altogether?

Troubleshooting a Flashing Fish Tank Heater

After setting the heater temperature, and fixing in the right position, it should heat the water by automatic process.

Even so, there will be times when a heater starts acting a little strange or it stops heating the water.

Common reasons why an aquarium heater flickers or shuts off, include:

Pro Tip: Fixing a blinking aquarium heater, or replacing fish tank accessories, is an important part of the fish care and pet fish health guidelines (e.g. a legal duty of care).

What Causes Aquarium Heater Light Flickering?

First, it's important to point out that a certain amount of heater light blinking is normal. Hence, it usually poses no risk to the fish.

But, if the sputtering becomes more frequent, or the red heater light fades out and stops working, then you should try to fix the problem.

Often, aquarium problem troubleshooting can be a matter of trial and error. But, check through this list of common causes of a flickering water heater light for a quick and easy solution.

Faulty Thermostat

The main job of the thermostat is to control when to generate heat, and for how long. You may have an issue with a faulty thermostat if it can't determine the actual temperature. As a consequence, it may start to blink (indicating that there is a temperature imbalance in the water).

Electrical Current Swings

Why is My Aquarium Heater Light Flickering On and Off?Many of the modern aquarium accessories use electricity when they run, such as filters and lights.

The current generated by these devices when they draw power from the mains can cause the heater to flicker, albeit a brief stutter.

Note: Another section focuses on troubleshooting Top Fin aquariums and fish tank accessories (e.g. pumps, filters).

Abnormal Heating Patterns

The frequency of a water heater turning on and off (called rapid cycles) may cause the LED indicator lamp to flash. In general, this is a warning sign that something is faulty.

Aquarium Heater Malfunction

Our fish aquarium beginners guide points out some of the pitfalls of buying a cheap water heater for a fish tank. In a nutshell, poor quality electrical devices have short lifespans.

A continuous flashing red light is usually a warning that it will stop working soon. Read through the manufacturer's operating guidelines (e.g. the Top Fin heater instructions) for further information about malfunctioning submersible fish tank water heaters.

Inadequate or Absent Power Supply

Despite being uncommon, an insufficient power supply is likely to create problems for all the electrical accessories inside a fish tank.

In other words, current fluctuations and momentary drops in mains supply can cause the aquarium heater to start flickering.

Normal Blinking of Aquarium Heaters

When you start a fish keeping hobby there are several easy lessons to learn. One of them is understanding that it's quite normal for the red LED heater light to turn itself on and off.

This is usually a good sign and indicates that it's heating the water to a preset temperature. It can also blink, shine intermittently, if there are slight changes in current flow.

Put another way:

The heater lamp glows red when it's working and it goes out once it's reached the relevant temperature. Thus, the internal thermostat will shut itself off when it no longer needs to heat the water.

But, you may have a problem if the red light flickers more than it should, or the blinking continues over a period of several days. If so, you may need to disconnect the heater from the power supply and investigate further.

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