Why Won’t My Aquarium Heater Turn Off? Reasons Explained!

A heater is an essential element of your tank. And the issue that comes with it is that it often won’t turn on. The rarer issue is that it keeps heating the water beyond the set temperature.

So that might lead to the obvious question, why won’t my aquarium heater turn off?

The reason why your fish tank’s heater is not turning off is as follows. Often since the thermostat has stopped working. Or that its functionality has drastically reduced. Apart from that, it may also be due to calibration issues with your heater. Apart from that, the heater will eventually stop working.

So let us dive into the article and see the possible problems and solutions.

Why Won’t My Aquarium Heater Turn Off? Three Reasons

heater in fish tank

Let us dive into this particular issue in detail then shall we? Firstly, there will be some context for heaters. This is followed by three possible reasons. The issue is almost always within the mentioned issues.

Reason 1: Difference Between What the Heater Has Been Adjusted to and the Actual Temperature

Some heaters will be off by a few degrees. For example, you end up setting the temperature at 80 but the temperature heats up to 84. In such cases, the heater probably isn’t broken.

Still, try to be careful of the aquarium heater overheating. 

Solution 1

You’ll have to use the help of your trusty thermometer. Find a sweet spot of the difference between the temperatures. Check the temperature difference between the heater and the thermometer.

 Adjust your heater accordingly. 

This can lead to another issue, and that’s related to properly calibrating your heater.

Reason 2: Heater Calibration issues

Heater calibration is setting your heater’s temperature to the desired level. This issue is that you may often find issues calibrating your heater.

This is a problem that will affect relatively higher-quality heaters too. The issue is that you might not have spun the dial to the desired temperature.

The thing is that these dials are made to be hard to turn. This is so that the dial doesn’t accidentally turn in the water.

So when does the problem occur? It’s when it feels like you need a set of pliers to successfully turn the dial. 

Solution 2

In such cases, the problem might be coupled with the issue mentioned above. In such cases, what’s the best course of action?

It would be to dial the temperature lower than the required temperature.

This will allow some room for possible damage control. If this still doesn’t work then the heater has probably served its time on earth.

Trying to make it worth further may not be worth possibly killing your fish. Even a few random spurts of higher temperature may cause sensitive fish to become stressed.

And we all know where that leads to!

One option that you might opt for is heaters that have a regulator outside the tank. In such regulators what might generally happen is that it comes with adjustable buttons.

This will take away the need for you to feel like you need a set of pliers or a wrench. The only downside to this is that these heaters tend to be very expensive.

Prepare to break the bank if you do opt for such an option. 

Reason 3: Faulty Thermostat

Secondarily it is probably because of the thermostat. Thermostats are what will regulate the temperature of your water based on the adjusted temperature.

The problem that exists is that it may end up being faulty. Another issue is straight up not working in the first place. This is a common occurrence with cheaper glass heaters.

As a result, your thermostat may stop functioning without any tampering.

Again, another cause is exposure to water.

The thermostat itself is not a waterproof component. The outer body of a heater generally is supposed to protect the thermostat. But there are possibilities of leakages, especially if the glass tubing cracks.

The issue with the glass tubing is that the crack might be untraceable. This causes the thermostat to stop working.

Unfortunately, there really isn’t a good way to fix it. Aquarium heater thermostats are generally not sold on a retail basis. Even if they are or you have a way of acquiring them you should still avoid it.

This is because the entire process involves a lot of dismantling and soldering. Issues that may leave you with burns or electrical shocks. Worse, it may exacerbate the issue or create a new reason for the thermostat to fail.

Solution 3

The best course of action would simply be to get a better heater as a replacement. We’ll be going through a few possible candidates later in the article. 

How To Check If Your Aquarium Heater Is Okay

fish tank heater checking

So, now that you know the reason you might want to know “How do I know if my aquarium heater is okay?”

Firstly, even though this might be obvious you definitely need a thermometer. In terms of the fish-keeping hobby, you should always think about it in a long-term sense.

It’s crucial to ensure that the thermometer is of good quality. Try to buy a decent digital thermometer.

Another point to look out for is to invest in a good heater. Buy the best heater that you can afford.

The best in the market are ones made of a titanium body and have external controls. Some possible combos include:

  1. Orlushy Submersible Aquarium Heater
  2. hygger 200W Titanium Aquarium Heater
  3. ZACRO 2 Pack Digital Aquarium Thermometer

The prices quoted are up to the first week of July 2022. Be careful of fluctuations in price. Check Hygger Brand Official website to know the latest price of necessary aquarium products.


How Do I Turn My Aquarium Heater Off?

You don’t need to turn your heater off if your outside temperature calls for a heater. Auto heater thermostats have you covered on the temperature of the tank 24/7. But just in case, you can simply plug it off. Another option is to use a timer plug. 

Can I Turn Off The Aquarium Heater At Night

You shouldn’t turn off the heater at night time. A heater will naturally fall back to its set temperature under ideal conditions. You can still opt for cooler temperatures depending on the species of fish. But adjustments mostly won’t be necessary. This is because temperature fluctuations may stress out your fish. 

How To Reset An Aquarium Heater?

Simply twist the dial to your required temperature. That should do the job. If you’re having difficulty then you can always ask for help. This is usually from LFS employees who are adept at adjusting temperatures. Most tanks will require constant temperatures anyway.


Why won’t my aquarium heater turn off? Well, there are three reasons as mentioned. They should help you in understanding why your tank’s heater is malfunctioning.

Make sure that you go through the guidelines that we have mentioned. Ensure that you buy the highest-grade items that you can afford.

Happy fishkeeping!

Howard Parker

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