How to Determine an Optimal Aquarium Heater Size

James Palmer
James Palmer
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Keily Brown
Keily Brown
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Last updated: August 17, 2023
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Fish are cold-blooded features that cannot produce their own body heat. Water’s temperature plays a huge role in sustaining their body temperature. If you have an aquarium you must ensure it is at the perfect temperature for your pets. You can only achieve this with an appropriate aquarium heater size.

What is the right heater size for your aquarium? Well, there are a few considerations you must make before purchasing one. First off, you must determine the size of your aquarium. How many gallons of water does it hold? Once you have established that you can get a heater with at least 3 to 5watts of power for every gallon of water you have. There are myriad different brands and models of heaters in the market. You can choose to use one or multiple ones depending on how you aquarium is set up. Most heaters have their heating power and capacity indicated by wattage.

Why get a heater for your aquarium?

If you have a freshwater fish tank, then you probably need a heater to sustain your fish. When it comes to aquarium’s temperature, there’s no one-size-fits-all. It varies with the kind of fish you are keeping.

If you have tropical fish, a heater is a must-have. You must ensure the water’s temperature is steady throughout and it doesn’t fall too low, especially in the colder seasons. That being said, always ensure your aquarium’s temperature is above 72 degrees F if you keep the likes of gouramis, tetras, neons, betta and cichlids.

If you keep cool water fish, then a heater won’t be a necessity. They are capable of enduring a wide range of temperature. In essence, cool water fish are hardy and can hold up well in just about any temperature. Goldfish, Bueno aires tetras, zebra danio, and bloodfin are some of the fish that can survive without a heater in the aquarium.

As for sub-tropical fish species, mostly from Southern China, Florida, Arizona, and Indian Northern highlands, you have to make the aquarium’s temperatures slightly lower. These regions experience warm summers and cold winters.

Types of aquarium heaters

There are multiple kinds of aquariums to choose from and you can use more than one if your set up allows it. They include:

Submersible heater

How to Determine an Optimal Aquarium Heater Size

This heater is the kind that is completely submerged in the fish tank or aquarium. You can mount it to the tank’s back either vertically or horizontally. Many people prefer to mount theirs horizontally near the substrate. If you have a large fish tank or aquarium, a submersible heater is your best bet because it provides more consistent and efficient heat.

According to expert reviewers, the popular Orlushy Submersible Aquarium Heater is one of the best submersible heaters that money can buy. It is a favorite go-to for many because it can be adjusted at small intervals.

In-line heater

An in-line heater is electric and it has a high output heat source. It heats fluids pressed through it. Most in-line heaters are self-contained units installed either inside or along external piping. Therefore, you must have external aquarium equipment like a water pump to move aquarium water through.

Some inline heaters come with a filter; either canister or power and they warm the water as it passes through the filter. They ensure that only clean and heated water end up in the fish tank. The CORISRX 150w/300w/500w in-Line External Heater is one of the most acclaimed in-line heaters. It is a top choice for many top reviewers and experts because it is easily adapted for use in various instances when heated water is required in an aquarium.

Undergravel heater

This is the best heater for freshwater planted aquariums. In essence, it consists of a long cable set in curves and placed at the bottom part of an aquarium. The cable is connected to a control unit. Undergravel heaters are more of microcirculation systems for the bottom of an aquarium or fish tank than water heaters. They only heat the fish tank but hardly increase the water’s temperature.

What size aquarium heater do you need?

How to Determine an Optimal Aquarium Heater Size

There are 3 main things that determine the size of an aquarium heater you need:

  • The water volume of your fish tank
  • The average temperature of the room in which your fish tank is located
  • The desired water temperature

To choose a wattage for the heater, stick to the standard rule of thumb that states you should use between 2.5 and 5 watts for every gallon of water in your aquarium.

This will help you get the correct heater size for your 20, 75 or 150 gallon aquarium. You can use more wattage, depending on how much you intend to increase the aquarium’s temperature.

Also, if you are using more than one heater, you have to ensure that all of them add up to the required wattage.

To get the amount of heating required, use the aquarium heater size calculator. Simply subtract the average temperature of the room from the desired temperature of the aquarium.

How to use an aquarium heater

Here are a few quick tips on how to use an aquarium heater;

If you have a large aquarium or fish tank, use two heaters. Install them at opposite ends of the aquarium for even heating. You can also use 2 heaters if the temperature of the room is significantly low.

Place the heater by the filter’s water outflow. This allows the moving water to disperse the heated water throughout the fish tank.

Ensure the tube length of your heater matches the height of your aquarium because heat rises.

Check for any sources of heat that can cause fluctuations in room temperature around the aquarium.

Use an aquarium thermometer to ensure the heater is working well to maintain the correct temperature in the aquarium.

Final thoughts

Temperature is one of the most crucial factors that determine the well-being of your fish. Since the fish rely on the aquarium’s temperature to maintain their body heat, you must ensure it is ambient enough. That said, you need to invest in the correct aquarium heater size. So, what size of an aquarium heater do you need? This depends on the water capacity of your aquarium, the desired temperature and the average temperature of the room in which the aquarium is located.

When choosing a heater, do keep in mind that most aquarium heaters range between 25 watts and 300watts. Therefore, if you have a small fish tank, you will be forced to make do with the 25 watt heater. We have provided some hacks on how to use a heater optimally.

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