Aquarium Heater Size Guide: What Size Heater You Need for Fish Tank

Animals living in your aquarium that have a low body temperature need the water’s temperature to maintain their body temperature. Heat is necessary to keep your aquarium’s proper temperature for your fish. You can use a single heater or a variety of heaters depending on your needs. Using the wattage rating of a heater is an excellent way to estimate its capacity.

Aquarium heater size guide

As in nature, temperatures drop by several degrees at night and during storms, most aquarium fish can withstand lower-than-ideal temperatures. The water should be kept consistently warm to reduce stress on your fish and prevent illness. Some species, such as Betta fish, goldfish, and Japanese rice fish, prefer lower temperatures, so heaters are unnecessary. Some species, including discus, ram cichlids, and Apistogramma cichlids, prefer warmer temperatures around 85°F, so heaters are necessary.

What is the Best Aquarium Heater in 2023

The availability of aquarium heaters has increased substantially in recent years, but I have experienced some heater failures. As a result, these might be stressful for you and harmful to your fish. Make sure you invest in a dependable heater to avoid these problems with your heating system.

A variety of aquarium heaters are available, including submersible, floating, and hang-on models. Furthermore, they come in different sizes depending on how big your aquarium is and how many different species you have.

How do I choose a fish tank heater? (Type & Size)

You need to pick a heater for your fish tank that matches how big your tank is and how strong the heater is. A good rule is to have 5 watts of heater power for each gallon of water in your tank.

Also, think about the different types of heaters for tanks. There are two main kinds: heaters that go inside the tank and heaters that connect outside the tank.

Most fish keepers use heaters that go inside the tank and can be put underwater. These are good because they work well, are dependable, and don’t cost too much.

Types of Aquarium Heaters

Heaters provide unique controls, regardless of their location within your tank. It is up to you which type of aquarium heater you use and how many you use.

Heating cable

Control units are connected to heating cables buried in gravel or substrate. It might be beneficial to remove dead patches from freshwater-planted aquariums. In saltwater reef systems, this type of heater isn’t ideal because you must dig up the substrate if the cable needs to be repaired or replaced.


A hang-on warmer usually attaches over the edge of the tank with suction cups or a hook. Although they tend to be less efficient than other types of heaters and are only partially immersed, they can still provide enough warmth in smaller tanks. If you intend to use multiple hang-on heaters in a big tank, it’s best to place them at opposing ends.


A submersible heater is installed either horizontally or vertically in the rear of the tank and is fully immersed in the water. The majority of them are positioned horizontally close to the substrate. With larger tanks, submersible heaters provide more reliable and effective heating than hang-on heaters.

Pro Tip

Read the included instructions before using your heater for the first time and, more importantly, before adding fish to your aquarium. Before adding fish to your aquarium, let the water warm up so that glass and water are both at the same temperature.

Choose the Right Aquarium Heater By Size

Choosing the right heater for a tank depends on how much water the tank holds, how warm the room is where the tank sits, and how warm you want the water in the tank to be.

For the power of the heater, a simple way to think about it is to use between 2.5 and 5 watts for each gallon of water in the tank. But sometimes, you might need more power if you want the water to get much warmer. If you’re using more than one heater, their combined power should match the needed power.

When using multiple heaters, their combined total heating capacity must equal the required wattage. The temperature of the water in your aquarium will depend on the type of fish you have, so ask yourself what temperature you want the water to be at before making your purchase. 

A heater that maintains 78 degrees Fahrenheit is necessary for tropical fish. If your fish prefer more temperate water, choose a filter that keeps the temperature between 65and 75degrees Fahrenheit.

When selecting an aquarium heater, be sure to consider both the aquarium’s size and the heater’s size. If you have a 50-gallon tank, you will need a heater of at least 100 watts (190 liters).


  • Average room temperature: 68 degrees F
  • Target water temperature: 77 degrees F
  • Heating required: 9 degrees F (77 – 68 = 9)
  • Tank size: 20 Gallon
  • Heater size needed: 50 watts
Aquarium SizeHeater Capacity
5-10 Gallons50 Watts
15-20 Gallons100 Watts
25-40 Gallons150 Watts
50-60 Gallons200 Watts
75-100 Gallons250-300 Watts

It’s really important to remember that this chart is only a general guide and that your aquarium may have different specifications. If you live in a very cold climate, the heater will have to work harder to raise the water temperature. This may require a stronger heater.

Each tank can hold onto heat differently. If you put a lid on the tank, it will stop the warmth from escaping. Glass tanks lose heat faster than tanks made of a different material called acrylic.

What about room temperature?

When you’re trying to figure out what size heater will work best, you also need to think about how warm or cold your room usually gets. The chilliest part of the room tends to be around 5 in the morning.

When you pick a heater, knowing how cold the room gets at its coldest is important. If it’s more than 5 degrees colder than the temperature you set on the heater, following the instructions that come with the product might not work very well.

For example, if you set the heater to 27 degrees and the room ends up being more than 5 degrees colder than that, the heater will have a hard time keeping things warm enough. In this case, it’s a good idea to think about getting a bigger heater that can easily make the room as warm as you want it to be.

Final Words

Picking the best heater for your fish tank isn’t hard if you think about it and look into it a bit. There are many types of aquarium heaters available, and you can find the perfect one for your tank at The Tech Den. We hope this guide has made it easier for you to choose the right heater for your fish tank.

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