Aquarium Heater FAQ and Troubleshooting

Last Updated on July 4, 2021 by cmoarz

A lot of people come to our site with questions about heaters, So this article will answer as many of those questions as possible so we can link them out to the people who need them.

If you found this FAQ useful, let us know in the comments and give it a share if you think you know others who will also find it useful. let’s get started.

Editors note: This article will continue to be updated with new questions as they come in.

How do I know if my aquarium heater is working?

First of all, read the instructions for your particular heater. If you can’t find them, call the company that made it or do a search on Google to find out how to use your heater properly. They’ll give you the information you need to determine if your aquarium heater is working properly and set up properly.

Next, If there is power going into the socket and it has been turned on for more than a few minutes, but it’s not getting hot, then you may have a heating coil problem.

Just double-check first to make sure it’s simply not turning on because the water temperature is already at the pre-set temperature, if it’s not, then it could be the coil.

A heating coil is an electrical resistance wire wrapped around a cylindrical insulator to create heat in water by starting and maintaining an electric current through the wire.

If you see any cracks in the glass, the heater should be considered dead (and dangerous).

This happens when the heater is turned on but hasn’t been fully submerged into the tank water. The glass heats up too rapidly instead of being able to dissipate the heat into the water. This causes your heater to crack.

This can happen quickly, even in the quick span it takes to do a 25% water change. Be sure to unplug or turn off your heater before doing water changes if at any point the glass is not completely submerged.

Can you fully submerge a fish tank heater?

Yes, most aquarium heaters will be designed for this type of use. You should consult the packaging and manual of your heater to double-check before fully submerging your heater as some do not fully submerge. An example of one that can not be fully submerged in most cases is a hanging heater.

Aquarium heater vertical or horizontal?

A full submersible aquarium heater can be put in either verticle or horizontal and comes down to personal preference.

Hanging heaters and other heaters not designed to be completely underwater should be placed as directed by the manufacturer. Usually vertically.

Does an aquarium heater need to be fully submerged?

Some heaters can be fully submerged, but not all of them. If your heater is designed to be completely submerged, it should come with instructions telling you how deep you need to submerge the heater in order to get the best results.

How deep should an aquarium heater be submerged?

If your heater is designed to fully submerge, you should follow the guidelines given by the manufacturer as these will help you get the most out of your heater and avoid overheating or even worse underheating.

If it’s one that is only meant to be partially submerged, there should be a water line indicator on the heater itself. The rule of thumb is glass should always be fully submerged and in contact with water at all times to avoid fracturing of the glass.

Where does the heater go in a fish tank?

Many heaters are designed to be placed on the floor of your fish tank, but they can also be hung or even placed at the bottom. The best place for a heater will depend on what type of aquarium heater you have (submersible/immersed vs hanging) and how powerful it is.

Generally speaking, submersible heaters will do better if they are submerged deeper, and more powerful hanging ones will need to be closer to the top surface.

Refer to your user manual for your specific model for specific instructions.

Can aquarium heater burn fish?

They can. If they get stuck behind the glass/heater element, they could potentially be burned. A lot of this comes down to the type of model you have. Prolonged contact on any heating element, even if it’s fairly low wattage can cause burns to fish.

If this is a concern, consider using an aquarium heater guard to keep your fish away from the element. The size and shape will depend on the model of the heater you have. Check if the manufacturer for your heater sells a specific model shield.

How do aquarium heaters work?

Aquarium heaters are an excellent way to regulate the water temperature in your tank. It’s important to know a little bit about how the heater works so that you can ensure you’re getting the best out of it and not running into any problems in the future.

Many aquarium heaters will have an on-off switch that is plugged into a power outlet. When you activate this switch, it will send a signal to the heating element inside of your heater, which causes an electric current to flow through it. This electric current is what creates the heat in your tank by starting and maintaining an electric current through the wire with resistances.

Most automatic heaters will have a sensor built-in that will determine the temperature of your tank and activate the heating element to make any necessary adjustments. Some even come with temperature control you can set yourself.

More expensive heaters may have a probe that is inserted into your tank in order to measure the actual temperature at a specific point in your tank. This can help you regulate the water temperature more effectively by making sure your getting a more accurate reading away from the heater coil.

Types of aquarium heaters

There are 4 main categories of aquarium heaters.

The in-line heater: This is for larger tanks with filters that sit outside of the tank. It heats the water as it comes into the tank rather than heating the tank directly.

The substrate heater: This is a type of heater for special use cases. They are usually wires that you layout at the bottom of your aquarium to help more evenly heat the substrate and the aquarium itself. Most people won’t need these types of heaters.

The filter heater: The filter heater is just a filter with a heater built-in. Nothing special.

The hanging heater: The hanging heater, aka an immersion heater, is the glass ones you’ve come to expect when seeing an aquarium heater. The glass is completely immersed into the water, while the top usually sticks out.

The immersion heater also comes in full immersion mode where only the cord will come out of the water. There are many variants of this heater and it is one of the most popular among users.


Owner of and also owner of actual Aquarium Gravel believe it or not! ;). Setting up beautiful aquarium sceneries and habitats since I was very young. Enjoy!