Aquarium Heater Size Guide

Finding the perfect heater size for your aquarium is essential for a thriving ecosystem. Unlike humans and other mammals, fish and reptiles are cold-blooded creatures that get their heat from the water’s temperature. Therefore, you need to provide enough heat to your aquarium’s water and make sure that it stays at safe levels at all times. While this might sound complicated, there is a bright side. There are lots of aquarium heater options that you can pick from, and today we are going to help you determine which size is the best for you. Check out our review of the best aquarium heaters for more detailed information on specific products.

What are the Types of Aquarium Heaters?

As previously noted, there are lots of aquarium heaters that you can pick from. They all work in different ways and are suited best for separate fish tanks. Check out the list below:

1st Hanging/Immersible Heater

Hanging heaters are known as immersible heaters in the aquarium community, and they hang over the top of your aquarium. The heating element of the device is encased in a glass tube, and it goes into the water. This type of heater is secured tightly with either suction cups, hooks, or other fastening devices that keep it stable. The biggest pro of choosing a hanging heater is that it does a fantastic job of providing warmth to smaller fish tanks such as nano. Not just that, but hanging heaters are easy to mount. However, this type of heaters is not very efficient when it comes to bigger aquariums.


  • control unit outside;
  • easy to assemble;
  • good for small aquariums.


  • not very efficient.
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2nd Submersible Heater

If you are looking for a heater that is easy to install and that doesn’t require any attention other than setting the right heat level, then you are going to love submersible heaters. They sit under the water, close to the inlet of the filter so that it can heat the water just as it comes back into the aquarium. This type of heater is best suited for larger tanks. In addition, the submersible heater can be mounted horizontally near the substrate so that it doesn’t ruin the view.


  • easy to install;
  • easy to use;
  • best for larger tanks;
  • not visible on the ground.


  • It is pricey.
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3rd Heating Cable

The last type of heater that you are going to find is called a heating cable. What makes this device stand out from the previous ones is that it can be placed under the gravel or substrate while keeping the control unit outside. Heating cables are best used for freshwater aquariums because they make it easier to get rid of dead spots. On the downside of things, you will need to get dirty when mounting the device. The reason behind this is that you need to dig up the substrate whenever the cable needs to be installed or replaced.


  • best for freshwater aquariums;
  • control unit outside;
  • reaches all areas.


  • complex to install.
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How to find the right Aquarium Heater Size?

Figuring out how to find the right aquarium heater size is a bit tricky if you are new to this hobby. Here is where we come in. The first thing that you need to take into consideration when searching for the right heater size for your aquarium is the water volume of the tank, the room temperature, and what is the exact temperature that you want the tank’s water to have.

Aquarium experts advise that the rule of thumb everyone should follow when searching for the best aquarium heater size is to use 2.5 to 5 watts per gallon. However, we do need to mention that bigger tanks might require more watts per gallon. If you want the temperature to stay at high levels, then you will require a powerful heater, and this type of device usually comes with more than 5 watts per gallon. The problem with this is that your electricity bill will be more expensive than usual.

Don’t forget to take into consideration the room temperature. This can be done by subtracting the room temperature from the desired temperature for your aquarium’s water. The result of the subtraction will show you the exact heating that is required to achieve your goal.

The best Places to put the Aquarium Heater

The biggest mistake that new hobbyists make when mounting their aquarium heater is not trying first to figure out what is the best spot. Finding a strategic place to install the device is essential because it will make sure that the entire body of water gets heated at the same time, and there are no cold spots. The best location is as close as possible to the water flow, or close to the filter inlet to ensure that the water is warm when coming back in. Doing this will guarantee that the heat produced is equally distributed to your fish by the current created by the filter.

If you are planning to set up a larger fish tank, then you are better off using two medium-sized aquarium heaters instead of a big one. The devices can be placed on opposite sides so that there isn’t a warmer or colder side. Your aquarium’s inhabitants are going to feel much better and livelier if the entire body of water is at the same level of warmth.

The last tip that we want to share about finding the best places to put the aquarium heater is to love enough space between the heater and the substrate. This is quite important because your fish can get stuck against the heater, and this is never good. You should also provide enough space between the heater and the sides of the aquarium if it is a hanging/immersible one.


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