Best 20 Gallon Aquarium Heater: Top Picks to Keep Your Tank Warm

An aquarium heater is a critical component of any freshwater tank. There are a variety of sizes, styles, and power based on the required factors.


To avoid confusion we have put together this review of the best heater for a 20-gallon aquarium. Finally, the one which made it on top of my list is the Eheim’s Jager 100 watts heater. Just a little below this you’ll find out why it got to be my personal favorite. Read on! 😉

Product Details
1. EHEIM Jager Aquarium Heater

Perfect Temp

  • 18-34°C
  • Lab Glass Jacket
  • Fully Submersible
  • Easy Setup
  • Fresh/Saltwater
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2. Aqueon Pro Adjustable Heater


  • 68-88°F
  • LED Indicator Lights
  • Auto-Shut Off
  • Various Wattage
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3. Cobalt Aquatics Heater

1 Touch System

  • +/- Range Of 0.5°
  • Dual Temp Display
  • Auto-Shut Off
  • Fully Submersible
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4. Aquatop Submersible Heater

Analog Design

  • 50-300 Watts
  • No Guesswork
  • Knob Adjustment
  • Freshwater/Marine
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Best 20-Gallon Tank Heater Reviews

1. Aqueon Pro Adjustable Heater, 100W

A well-built submersible heater that offers consistent tank heating.

The Aqueon Pro is a highly durable heater that is shatterproof and is fully submersible. The automatic shut-off engages to prevent overheating and will re-engage once the unit has cooled down.

The design of the Pro heater is made to last. The manufacturer is confident enough in their product that they offer a limited warranty on the lifetime of this product. An LED is always working, with the color red indicating the Pro is heating the water and the color green highlighting that it is not. The adjustment may prove to be highly sensitive and require some effort to get the exact levels you desire.

This heater may be bumped or nudged off of settings more easily than other models.


  • Limited lifetime warranty
  • Highly durable design protects against cracking and shattering
  • Continuous LED lighting with red for heating and green for idle


  • Adjustments for temperature settings can be sensitive and require multiple adjustments to “dial-in”


2. EHEIM Jager Aquarium Thermostat Heater 100W

A strong heater that is loaded with extra safety features.

Eheim’s Jager is designed for durability and comes with features to help extend its lifetime. The glass is shatter-proof as well as shock-resistant. A simple on and off indicator allows you to view the product’s status with a single look.

The Jager will shut off automatically if it senses that the water level is too low. This submersible heater has overheating safety controls that will aid in a low water level shutdown. The TruTemp dial allows you to recalibrate the temperature settings, which can help you get the exact level you need.

The temperature controls can be sensitive, requiring more work to calibrate the desired temperature. This heater attaches to the aquarium wall by suction cups.


  • A thermal safety control protects this heater from running dry
  • TruTemp dial will recalibrate the heater and offer more precise temperature adjustments
  • Glass is both shock resistant as well as shatterproof


  • Due to the ability of recalibration on this heater, settings may need extra work to set at the desired level
  • The product may be a bit bulkier than other heaters of similar wattage


3. Cobalt Aquatics Flat Neo-Therm Heater

A highly durable heater with extra built-in safety features.

The Neo-Therm is fully submersible and built in a modern, flat design. A built-in LED display shows both the temperature settings and water temperature at the same time.

This heater has a temperature range from 66 to 96 degrees Fahrenheit. The controls offer easy access with a one-touch control system, which can make adjustments easy from anywhere in the tank. Added thermal protection on the circuitry shuts the heater down before it overheats, which can extend the life of the heater.

The freshwater and saltwater capabilities will help to prevent this product from rusting or corroding over time. The 100-watt system was designed for tanks up 24 pounds, so this should provide more than enough power for a 20-gallon tank.


  • A highly durable outer casing makes this shatterproof
  • Higher temperature range at 96 degrees


  • The product comes at a higher price point than some heaters at this wattage


4. Aquatop Submersible Glass Aquarium Heater

The Aquatop Heater will keep your aquarium in just the right temp for your Betta. Made with high-quality glass and analog design that makes it even simpler to use with just a turn of a knob for temperature adjustments. No-fuss setup too, with the suction cups that are included with the package.

This Aquarium heater is available from 50-300-watts for various fish tank sizes.


  • Perfect for freshwater and marine aquariums
  • Clear, analog temperature display
  • Easy setup


  • Temperature control readings can vary from the actual water temperature


Some Notes About 20-Gallon Freshwater Aquariums

The 20-gallon aquarium is possibly the most popular sized tank in the fish-keeping hobby. Freshwater tanks of this size come in a variety of shapes that offer the owner more options for display and function.

Due to their larger size, these tanks are able to house more fish as well as fish of a larger size. You need to select a heater for your tank that is effective and safe.

Water Temperatures And Fluctuations In A 20-Gallon Aquarium

Fish are classified as poikilotherm, which means that they are not capable of generating their own body heat internally. Due to this, they must be able to gain internal heat from outside of their body. This heat is absorbed by the water that they live in.

A good rule of thumb for temperature ranges in a freshwater aquarium will range between 71 to 86 degrees Fahrenheit.

This temperature range will be finite with particular species of tropical fish. The water volume in a 20-gallon tank will not fluctuate as quickly as the water in smaller aquariums with heaters. Even so, the temperatures in these tanks can lower and raise with fluctuations in your home.

Consistent temperatures prevent your pets from becoming stressed. Water temperatures that become too high or low can affect the health of fish and may even kill them.

Heater Power Or Wattage Ratings

The ability of a heater to control water temperature is based on wattage. The higher the wattage rating, the more powerful a product will be. In general, heaters below 50-watts are not powerful enough to maintain proper temperatures while heaters greater than 100-watts are too powerful.

A basic formula for calculating heater wattage is to use 2.5 to 5 watts for each gallon of water. A tank with a lot of space taken up with decorations will take less wattage per gallon and a barren tank will usually take more. Room temperatures also factor into this, as colder rooms will require more wattage to maintain water temperatures.

The Number Of Heaters Used

Often, owners may be tempted to use more than one heater of lower wattages to maintain temperatures in tanks larger than 10-gallons. Some enthusiasts also do this for safety reasons, using a second heater as a backup.

A 20-gallon tank is still small enough that a single heater should be sufficient. A single heater of the proper wattage will efficiently maintain water temperatures evenly.

Multiple heaters in a 20-gallon tank may also keep the temperature too high for the fish. Also, space will still be a consideration for a 20-gallon tank.

 Types Of Heaters Available For A 20-Gallon Aquarium

There are different styles of heaters available to use with freshwater aquariums, and some are more or less effective than others.

Submersible Heaters

Because these heaters can be fully submerged, they do not take up room outside of the tank. Many products are available in the desired wattage range that will take up a minimal amount of space. These heaters are considered by many to be the best option for a 20-gallon freshwater aquarium.

Immersible Heaters

The next best choice for a tank of this size may be this style of heater. Also known as a hanging heater, the heating element and sensor will be submerged while the rest of the unit hangs off of the side of the tank. While this does take up more room on the exterior than a submersible, it is still more compact than other designs.

In-Line Heaters

This design is located outside of the tank between it and the filtering system. It will take up more exterior space, which may still be a consideration for you if you have limited room. The range that these products are offered in is often 100-watts or greater.

In-Sump Heaters

These units are located in the filter/pump system. They may be too large for a 20-gallon filter set-up. They are usually produced at levels greater than 100-watts.

Substrate Heaters

This design is usually a cord buried in the substrate. Due to this, they are harder to maintain than other types of heaters. The power levels for most heaters of this type are 150-watts or greater, which is too powerful for a 20-gallon tank.

The Best Heater For 20 Gallon Aquariums From Our Reviews

After reviewing the previous five heaters, we feel that the​ Eheim Jager Submersible Aquarium Heater 100 Watt offers the best combination of features.

The durable design, including the shock-resistant and shatterproof glass, is important. The size is adequate to keep a 20-gallon aquarium warm. Even though the temperature controls can be sensitive, they allow you to precisely adjust the temperature.

The safety control features that guard against damage from accidental dry running are superb. After reading this, we hope that you agree and perhaps give it a try!

Last Updated: October 17, 2021

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