Best 20 Gallon Aquarium Heater

Eheim Jager 75 WattMarineland Precision 100 WattHydor Glass 100 WattAqueon Pro 100 Watt
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3 Year WarrantyLifetime Warranty2 Year WarrantyLifetime Warranty
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20 Gallon aquariums fall between small and medium tanks. While it is true they double a standard 10 gallon, they also often introduce larger rocks, more decoration and more equipment. All of these can reduce the actual water capacity, introducing a new element to the fish keeping hobby.

Still with 15 to 20 gallons of water to heat we need a bit bigger of a heater. Thankfully many of the best companies make heaters that are perfect for our 20 gallon fish tanks. I’ll start off with my absolute favorite, which also happens to be the most reliable 20 gallon aquarium heater.

Eheim Jager 75 Watt

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  • Comes In 75 Watt Size
  • Ultra Efficient Heating
  • Extreme Lifetime
  • Specialized Glass


  • Bulky
  • Old Fashioned Look

The Eheim Jager has been one of the best aquarium heaters for over a decade and shows no sign of changing. Made by their original manufacturers in Germany, This heater comes equipped with multiple safety features, accurate temperatures and of course the special laboratory glass casing.

The special glass has three major benefits that other companies can’t seem to figure out. First off is the perfect, fissure and crevice free surface. Any cracks or abnormalities result in a shortened lifespan. Because the Eheim prevents these it will never shatter or crack accidentally.

Furthermore the glass is unrealistically strong. My inverts have, on multiple occasions, caused rock slides to fall onto my horizontally placed heater. Each time the heater showed no signs of damage, even though a couple pieces of the rock chipped off.

The specialized glass also increases heating efficiency to lower operation costs while creating a heat shield. Even while at full power the heater will not harm any live stock, including those curious snails who love climbing all over warm heaters.

Settings & Safety features

Adjusting the Eheim Jager is nothing special. Simply turn the blue ring located on top of the heater to your desired temperature. There is a red arrow pointing at the wheel, making it easy to set an exact heat

Should the heater come in contact with air, either from water evaporation, water changes or moving the heater, it will shut off automatically and resume upon being fully submerged. This helps prevent any heater explosions, which are a real danger not to be taken lightly. I’ve had a cheaper heater shoot shards of glass at me when I was carelessly heating multiple buckets of water.

When the lights orange light turns on the heater is currently heating the water. This makes a light clicking noise. The Eheim Jager 75 watt measures at 10.63″ x 1.42″ which is a little long for a 75 watt heater. So long as it fits in your tank this is may top 20 gallon aquarium heater recommendation.

Lifetime & Warranty

When it comes to warranty Eheim takes all matters seriously and is happy to help. They feature a solid 3 year warranty and stand by it.

You likely will not need the warranty however, as the Jager has the lowest failure rate of any 20 gallon aquarium heater. Given normal circumstances the Jager at least 7 years, with many users reporting 10+ years and a couple at 20. Mine has been going for over 5 years and still looks/works like new. This is seriously the most dependably medium sized aquarium heater.

Marineland Precision Heater

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  • Comes In 75 Watt Size
  • 1 Degree Increments
  • Easy To Read
  • Powerful Mounting Bracket
  • Efficient Heating
  • Lifetime warranty


  • Shorter Lifetime (1-2 years)
  • Light Outlasts Heater
  • Can Trap Bubbles

Marineland, not exactly the brand you think of for heaters and more for the rest of the tank. However this low cost 20 gallon aquarium heater really brings a lot more to the table than expected.

Setting the temperature with the Marineland is easy, as each click changes the heat by 1 degree. This activates the mica core, which is surrounded by a mesh barrier that helps transfer heat and increases structural integrity. The heat settings range from 65 to 89 degrees.

Setting Up & Safety

Mounting the Marineland Precision heater is a bit different, as the three suction cups are attached to a mounting bracket. The heater rests inside this rather than having the suction cups directly on the heater. This gives a bit more exposure to the heater while keeping the suction cups more stable.

When first submerging the heater and bracket, rotate the two separately and release any and all air bubbles that may be trapped in the two. These can cause premature damage otherwise. This does not mean you cannot place the heater near bubbles, ad the trouble maker bubbles are only located deep in the heater.

This 20 gallon aquarium heater is fully submersible and comes with a 6 foot power cord, easily letting you hide the heater while still forming a drip loop with the cord. The heater is 10″ long and should not be touching the substrate.

The Marineland Precision heater features a thermal fuse which will shut off the heater when exposed to air. This keeps the heater safe when doing water changes or when moving it around.

Lifetime & Warranty

The Marineland precision heater lasts between 1-2 years, which is pretty good at the amazingly low price. After the heater gives out you can use the lifetime warranty by contacting Marinelands support at 1-800-322-1266. When you call you will need the code on the back of the rubber cap where the dial is. With my lights I have always received excellent customer service from their representatives, making Marineland my favorite on warranties.


The Jager and Marineland are special as they are the best heaters to come in a 75 watt size. The 100’s listed below are 100% safe, but they can cost a bit more to run.

Hydor Theo Submersible Glass Aquarium Heater

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  • Easy To Hide Colors
  • Strong Suction Cups


  • Does Not Come In 75 Watts
  • Weaker Heating Element
  • Medium Lifetime

The Hydor is a fairly low cost heater that does exactly what you expect with a few downsides. Most notably it does not work well in cold environments and costs a bit more to run.

Set Up & Safety

Located on the top of the heater is an extra thick rubber top. This is in place to prevent any shocks caused by light cuts in the heater. On top of that is the easy to read red dial, which ranges from 67 to 91 degrees Fahrenheit. Because the heater is a little weak I highly recommend using a second thermometer to keep an eye on the tanks temperature. Cold houses may need to set the heater much higher than actually needed, or choose a different heater entirely.

The heater is mounted by two suction cups, which have pretty solid grip. This is nice for mature marine tanks who’s walls often have coralline algae and need extra grip.

The big plus for the Hydor Theo is the extra compact design. The 75 watt is just under 9″ which allows us to use this in nearly any sized tank and easily hide it behind rocks or plants. This is a great plus as it gives the tank a more natural appearance than other heaters.

While the heater does say it shuts off when it is dry many users have not found this to be the case. Take a little extra caution here and treat the Theo as if it does not have auto shut off.

Lifetime & Warranty

The average lifetime for a non defective Hydor Theo is 2-3 years, Combined with the two year warranty gives you an average of 2-5 years of lifetime. This is the shortest of my recommended heaters, but the Theo has its own special merits.

The warranty is not as easy to use as the other heaters and requires a bit of emailing around. Expect this to take 2-5 days.

Aqueon Pro Heater

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  • Lifetime Warranty
  • Hardened Plastic, No Glass Components
  • Aluminum Core For Increased Durability and Heating


  • Setting Is Black On Black And Hard To Read
  • Temperature Calibration Is Often Off A Few Degrees
  • Does Not Come In 75 Watts

First let me give you a warning. Aqueon does sell a cheaper glass heater, however it is made in china unlike this one which is made in Italy. While some made in China products are amazing the cheaper version here is not.

Anyways the Aqueon Pro heater is a fantastic piece of equipment any aquarium would be happy to have. It features a strong, hardened plastic casing which makes it completely shatter proof. At best you will manage to scratch or dent the heater over the life of your tank.

Inside the heater is the aluminum core, which both increases heating efficiency while also increasing durability. The hardened case wouldn’t mean a whole lot if the insides constantly broke.

The two nice features on the Aqueon pro are its red and green light and auto shut off feature. When removed from water the heater will shut off automatically and resume when resubmerged. Fairly common in most heaters but it’s still nice to be sure. The lamp will either show green for correct temperature/off and red for low and on. Very few heaters actually have the confirmation light and simply switch off.

The two big draw backs are the black numbers on a black surface and the accuracy of the heater. When setting the heat you will either need a flashlight directly on the light or to have the heater directly under your lighting fixture.

As for the temperature accuracy the Aqueon Pro is often off by +/- 2 degrees, meaning you will need a second thermometer to set this heater. Adjust the heater until you have the right temperature and you’re good to go.

Lifetime & Warranty

With the average lifetime of the Aqueon pro being just above 3 years you should more than get your moneys worth from a single heater. This however will not be the case thanks to Aqueons lifetime warranty. Even if you get the golden heater that dies after 30 years Aqueon will happily send you a replacement once the heater does die off. No more looking for purchase dates with this heaters warranty.

Looking for other ideas on aquarium heaters? check out my more generalized page on the best aquarium heaters.

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