Do Aquarium Heaters Turn Off Automatically? (Or Always Stay On?)

Image of an aquarium with its heater turned off

Yes, aquarium heaters turn off automatically once the set water temperature is achieved. They will then turn on again once the water starts to cool down. However, heaters also turn off automatically due to malfunction, if the water level is low, wiring is faulty, or the thermostat is damaged.

Let’s understand this in more detail now.

Do Aquarium Heaters Stay On All The Time?

Aquarium heaters don’t stay on all the time.

They have an internal thermostat that turns off the heat when the water reaches the set temperature.

So you don’t have to keep track of the water temperature once you install the heater.

All you need to do is set the required temperature that suits the tank inhabitants and leave the heater plugged in all the time.

Since a thermostat controls the heater, it will switch on and off as required.

The only time you will have to switch the heater off is while performing a significant water change.

It’s essential to switch off the heater in this case because the water level during the water change will drop below the safety level marked on the heater.

Moreover, the heater will no longer remain fully submerged, and there’s a possibility that it may run dry.

Also, it’s best if you unplug the heater before performing the water change.

You can plug the heater back and set the desired temperature once the water is replaced.

For the additional safety of the fish, you can have a thermometer in the aquarium to monitor the temperature and ensure that the heater is working correctly.

The water temperature must remain stable all the time.

Should You Turn Off The Fish Tank Heater At Night?

You shouldn’t switch off the fish tank heater at night. It needs to be on all the time except while performing a large water change.

Fish need a consistent temperature to stay healthy.

If the heater is switched off at night, there will be temperature fluctuations.

Any drastic or sudden variation in the temperature is dreadful to the fish.

It causes stress and other health-related issues to the fish.

Stress for a prolonged duration even results in the untimely demise of the fish.

Alright! Now that you know the aquarium heater needs to be plugged in all the time, let’s also understand the instances when an aquarium heater will switch off on its own without attaining the set temperature.

When Does An Aquarium Heater Switch Off Without Heating The Water?

As we know, aquarium heaters switch off automatically once the desired temperature is reached.

So the switching off of the heater is not alarming in such instances.

You can see a green light before the heater switches off, indicating that it has reached the set temperature.

However, if you notice that the heater is not working, it must be thoroughly checked.

You can test the water temperature with a thermometer.

If the water temperature is the same as the set temperature, the heater is functioning correctly.

Conversely, if the water is cold, there’s a problem with the heater.

Below are some of the reasons why a heater will switch off without heating the water.

1. The thermostat is damaged.

The most common reason for the malfunction of a heater is that the thermostat has got damaged.

This can happen due to improper installation or if the heater is overpowered for the tank.

In such an instance, the heater will remain switched off until the damage is repaired.

2. Water level too low.

There are different types of heaters like submersible, immersible, in-filter, in-line, substrate, etc.

Accordingly, you need to install the heater either in the aquarium or outside it.

For example, if you have a submersible heater, it needs to be submerged inside the water for proper functioning.

If the water level drops below the safe zone, the heater will turn off until the water level rises above the safe zone.

This is because the heater is designed to work safely under these conditions.

3. Wiring is faulty.

It’s also possible that the wire inside the heater is faulty. In such cases, the heater will remain off until the faulty wiring is replaced.

You can take the help of a professional to resolve this issue.

4. Battery power is low.

Another possibility is that the battery powering the heater is exhausted.

In such instances, the heater will remain shut down until the battery is recharged.

Ensure that the heater is plugged in correctly and the outlet is discharging power.

If the problem persists even then, you can try to plug the heater in another outlet to check if it starts functioning.

5. The volume of water is high for the heater capacity.

You should choose a heater keeping the aquarium size in mind.

If you have a large tank with a high volume of water, the heater capacity should be of the correct wattage to heat the water.

Secondly, you need good water circulation in the tank.

Water flow is crucial because the heater will automatically switch off once the surrounding water attains the desired temperature, irrespective of the rest of the tank water temperature.

6. Heater is malfunctioning.

There’s a possibility that the heater is malfunctioning.

In such instances, the heater will remain switched off and will not heat the aquarium water.

Given below are some of the signs that indicate the heater is not functioning correctly:

  • The water is not warm. This is a clear indication that the heater is not functioning.
  • The heater doesn’t stay on for long. If you notice that your heater doesn’t remain on for a prolonged duration, there’s something wrong with it.
  • The lights of the heater keep on flickering. This is not good as it means the water temperature is fluctuating rapidly. It may be due to some problem with the power supply or the electrical components of the heating element. Another reason for the light to flicker is that the heating element is cycling too rapidly.

How Long Does It Take For An Aquarium Heater To Heat The Water?

The average time taken by an aquarium heater to heat the water is around 30 minutes to an hour. However, it’s not standard as it varies depending on the type of the heater and its capacity.

Some of the other factors that determine the time taken by a heater to heat the water are:

  1. The size of the aquarium,
  2. Water volume in the aquarium,
  3. Temperature of the room,
  4. Size and capacity of the heater,
  5. Heating element type, and
  6. The set temperature.

If you set a high temperature on the heater, it will take longer to heat the water.

Similarly, a large aquarium with a high volume of water will take more time to heat the water than a smaller aquarium.

Moreover, it will take longer to heat the water if the room temperature is cooler than when it’s warmer.


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