Best Heaters for a 5 Gallon Freshwater Aquarium Fish Tank: Review

Betta Fish
Betta Fish | Source

Always Use a Thermometer Inside the Tank

No matter what type of heater you ultimately decide upon, you need to always monitor the temperature. Stick-on thermometers that mount to the outside of the tank are recording the ambient room temperature and not the water temperature which can have a very different reading. For the most accurate water temperatures, use a glass thermometer that mounts to the inside of the glass with a suction cup!

Are you considering purchasing a new aquarium heater for your small freshwater tank?

We recently purchased a 5 gallon aquarium which would become the new home for my daughter's Betta fish. While we wanted to upgrade him from the temporary bowl she was using to a larger home, more importantly we knew that with the fluctuating evening temperatures we needed to get him into an environment that included a heater before the water temperature got too low than the tropical temperatures that we needed to maintain.

In the evenings, the ambient temperature inside our home began to drop to about 70 degrees. We rarely use heat in the house as we live in Florida, but in an effort to keep the bowl warmed, we were running the heat at night to maintain a 76 degree room temperature which would help keep the Betta at the lower end of the tropical temperatures he is meant to be kept in. As an additional precaution, we wrapped the bowl in thick towels to help insulate it as much as possible.

While the new tank was cycling, I began researching the aquarium water heater options and was getting even more confused with all that was out there.

There were a few things I knew I wanted in a small aquarium heater:

  • Small size
  • Internal thermostat
  • Low wattage for small tank
  • Submersible

Aside from those features, I was clueless. After reading more reviews of different small fish tank heaters than I can remember and after posting on a fish forum without any responses, I set off to the local big box pet store looking for a solution from the advice of the store employees.

Betta Bowl Heaters Can Kill Fish

While companies do manufacture heaters that indicate they are safe to heat small 1-gallon fish bowls, they're not safe. After much research on what could have been a simple solution, I learned that these flat, round bottom heaters can literally fry a fish as can be read in the numerous reviews on sites such as Amazon. Although they may be 7.5-watt heaters, they're not reliable and certainly not for use with a 1-gallon bowl.

How to Determine Fish Tank Heater Wattage Needed

A good rule of thumb to determine what size heater you may need for your small aquarium is:

5 Watts for every 5 Gallons

This is not an absolute rule as different heaters may work more effectively than others, but this will help guide you to finding a heater that is better suited for the size tank you need.

Not All Low Wattage Aquarium Heaters are the Same

Since we were upgrading to the 5-gallon tank, I did not even consider a temporary solution such as the Betta Bowl Mini Heater made by Zoo Med. While companies may advertise they're safe, a heater in anything as small as a 1-gallon bowl is too dangerous for any fish, especially if you can't maintain a tropical temperature. I would not advise using these at all, unless you're willing to sacrifice your fish.

During my research, many of the reviews I read suggested a 25-watt heater. So, I went looking for one at my local big box pet store. What I found was that some 25-watt heater labels indicated they were for up to 5-gallons, some were for no less than 7 gallons, but no more than 10 gallons. It got so confusing and frustrating!

On the market, there are 7.5w, 10w, 15w, 25w aquarium heaters- all indicating that they will work just fine for a tank 5 gallons and under. BEWARE! Not all low wattage heaters are the same!

After looking carefully at all the packaging and sizes and wattages, I picked up a 25-watt heater, a Top Fin® Submersible Aquarium Heater, which, according to the box was advertised for tanks up to 5 gallons. It had an internal thermostat, was adjustable, and fully submersible. It wasn't quite as small I was looking for, but we could have made it work. When the store employee asked if I needed help, I explained what I was doing and she said that the Top Fin® heater I had in hand was too powerful.

She handed me an Aqueon Mini 10 Watt Heater. And, so I gave it a try.

Aqueon Mini Heater (10-Watt) Review

The Aqueon heater was inexpensive enough (about $14) and I had several weeks to try it before the Betta would be added to the tank, so I decided to give it a try.

The features of the Aqueon Mini Heater 10 Watt included:

  • Fully submersible
  • Shatterproof
  • Thermal safety shut-off
  • Preset temperature to about 78 degrees
  • 10-watt
  • Small size - about 8 inches in length
  • Rated for up to a 5 gallon tank

The heater came with two mini suction cups that were easily attached. The installation was simple - put it in the tank, slightly push to engage suction cups, then plug it in.

After several hours, the temperature of the water increased from about 74 degrees to about 78 degrees where it maintained most of the time. I installed the heater on the side close to where the bowl with the Betta was and touched the side of the bowl to the side of the tank. The Betta hung out there as he could obviously feel the warmth from the heater through the glass.

This would have been the heater that I kept, except for one small issue. It indicated in the manual that if the ambient temperature reached 78 degrees, to unplug the heater. Since we were still cycling without the fish, we could test what happened when the room temperature increased. Here in Florida, it's not unusual during the winters for us to have outdoor temperatures in the 40s in the morning and 80 in the afternoon. What we found was on the warmer days, the tank's temperature was increasing up to 86 and at night was dropping to about 76-78. The room temperature was about 78 and 69 respectively.

We decided to look for another solution for a few reasons:

  1. Nearly 10 degree temperature swings in the water on the warmer days
  2. Someone is not always home in the afternoons when the temperature is much warmer to unplug the heater to prevent the water from overheating the fish
  3. Concern was also there that someone would forget to plug it back in

In climates where the winter temperature doesn't fluctuate as much and a home is heated consistently throughout the day and night to maintain a comfortable temperature, I believe that this would be a good solution that would work well.

Top of Dial
Top of Dial | Source
Submerged view of heater
Submerged view of heater | Source

Hydor Theo 25 Watt Heater

With the concern of the fluctuating temperatures using the Aqueon Mini 10-watt heater, I did some more research and even asked a question here on Hubpages. My research and the answer given resulted in a common product - the Hydor Theo 25-Watt heater - an adjustable heater for a 5 gallon tank.

None of our local big box stores, nor any of the several local fish stores carried this, so I ordered it on Amazon.

The features of the Hydor Theo 25 Watt heater include:

  • Fully submersible
  • Horizontal or vertical positioning
  • Auto safety shut-off
  • Shatterproof
  • Shock resistant
  • Adjustable temperature with 1-degree click graduate scale
  • 25-watt
  • Small size - 7 inches in length
  • Rated for 2 to 7 gallon tanks
  • On indicator lamp

The tube of the heater is approximately inch in diameter which was thicker than the Aqueon, but it came with two large suction cups that attached to C-clips which, in turn, clips to the Hydor Theo and holds in place in either the horizontal or vertical positions. In my tank, it is installed vertically with the dial completely submerged.

The temperature dial is located at the top end of the tube and stands about 1/2 inch high. It is easy to turn with clearly marked degree increments.

I love that the tube has an "on" light indicator which you can easily tell that the heater is on. I have an internal glass thermometer inside the tank on the right side (about ten inches away from the heater) and I can easily see that the tank is maintaining a consistent temperature.

I originally had it set at 80 which maintained right at 80 except the warmer days which would creep up to 82-84. I've since changed it to 79 which maintains right around 80 no matter what the room temperature is.

I tested this heater out for about a week while the cycling finished and was very impressed with the results. The Betta fish, who was more lethargic in his temporary bowl due to the colder water temperatures, was introduced to his tank and thrives now in his new home. After nearly two months in his new heated tank, he continues to be very active swimming and blowing bubble nests!

I highly recommend the Hydor Theo 25 watt Heater for a 5 gallon tank!

Adjustable Temp?
Temperature Range?
Adjustable 71 - 89 Deg
Preset to 78 Deg
Degree Incements
1 degree
Fully Submersible?
Shock Resistant?
Horizontal or Vertical Positioning?
"On" Indicator Lamp?
Tank Rating
2-7 Gallons
Up to 5 Gallons
7 inches
8 inches
Control Steady Temp?
Yes. Controls water temp at a steady temp even with varying ambient room temps. Good for climates that may have wide temperature swings during a day leaving inside temps at or around 78.
No. When room temp rises above 78, instructions indicate to unplug as it will continue to heat the water above the set 78 Deg. Best for climates in which indoor temps remain constant.

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Comments 14 comments

Ann1Az2 profile image

Ann1Az2 3 years ago from Orange, Texas

Thanks for the info! My fish tanks are all bigger than 5 gallons, but this is very useful information for someone who wants to set up a small tank. Very well researched an thought out and you were able to communicate your findings very meticulously. I enjoyed this hub! Voted up!

EricDockett profile image

EricDockett 3 years ago from USA

Good for you for using a heater in your 5-gallon Betta tank! A lot of people don't realize they need warm water just like any other tropical fish. Hopefully many Betta owners will read your Hub and follow your advice. I'm sure he'll be very happy in his new home. Way better than a bowl!

mypetfinderph profile image

mypetfinderph 3 years ago from Philippines

It is indeed important to maintain the right temperature of your fish tank. This article gives us a good idea on what heater to purchase for our aquarium.

sgiguere profile image

sgiguere 3 years ago from Marlborough MA

Voted up! Personally, I hate being cold and I would feel terrible if my little fish were cold. I linked your Hub to my Hub on care and feeding of a happy betta fish :)

Jamie Brock profile image

Jamie Brock 3 years ago from Texas

Interesting info here regarding heaters for small tanks. We recently bought 4 black molly fish (1 black, 1 white and 2 dalmatian) for a 10 gallon aquarium and 3 of them died.. I didn't realize some heaters worked better than others. Not sure what the brand of heater is on this tank. I wonder if that could have had something to do with the fish not making it. Also, you mentioned the Betta being more lethargic when it was just in the bowl. We have a Betta and it was originally in the 10 gallon tank but we took it out when we got the 4 black mollies and put the Betta in a large bowl. I noticed right away he was not near as active in the bowl.. in fact, for a while I thought he may be getting sick. I think I may need to get him a 5 gallon tank like you are talking about here. Great hub.. thanks for sharing!

teaches12345 profile image

teaches12345 3 years ago

We used to have an aquarium and found heaters to be very important to certain fish species. Your post is wonderful for those who need the resource information. Very well done.

KDeus profile image

KDeus 3 years ago from Florida Author

@Ann1Az2, @EricDockett, @mypetfinderph, @sgiguere, @teaches12345 - thank you! My daughter's little guy is much much happier now! I hope that my experience trying to find the right solution for a small tank will help others in the future!

KDeus profile image

KDeus 3 years ago from Florida Author

Jamie, I'm sorry to hear about the mollies. It could have been caused by any number of things - fish that weren't as healthy when they were at the store, temperature, ph, or high ammonia or nitrites. Do you have a water test kit?

It was so interesting when we were cycling the tank - the bowl was pushed up right against the tank to absorb some of the warmth and he was hanging out right there where he could see the tank but couldn't get to it. Once he got there, he does nothing but swim and make bubble nests. I, too, was worried that he was sick when he was in his bowl, but I think a lot of it had to do with the colder water since we were regularly doing water changes and the test kits showed no problems.

I hope this helps you if you decide to get the 5 gallon for your Betta!

poet4u 2 years ago

Hi, read this article and am looking for a reasonably priced aquarium heater for a 3 gallon Fluval that has an adjustable thermostat. I want to keep the water about 70. I live in so california and the water stay in 60's and can go lower so i want to heat the tank. Any suggestions on where I can get one and the best deal? None of the pet stores here carry them. Thanks :)

Poet4u 2 years ago

By the way my fish is a Betta and I just posted on a heater for my 3 gallon Fluval. I think I am going with the Hydro theo 25 watt heater after reading the review but wanted to make sure it was not too big. I am open too to other suggestions. I want it to take as little room as possible. :)

4103.5 2 years ago

Aqueon is not 8 inches long, it's actually 4 inches long... A very small heater if you have a small tank..

Tim Hiland profile image

Tim Hiland 2 years ago

Very nice work on finding the right heater for your Betta Sp. (hybrid). I am a non-game fish expert in North American fishes. I went to the corporate stores and they did not have a single adjustable heater for small tanks. I have read on talk forums etc., that Betta fishes need a heater that will maintain a warm temperature of 75-85 degrees F. This makes perfect sense when one considers where these GMO (Genetically Modified Organisms) fishes have evolved in shallow warm anoxic water such as: rice paddies, ephemeral swamps, and shallow bodies of water that do not have conditions suitable for predator fish and hence an adaptive radiation of these wonderful fishes has taken place in the tropics. Unfortunately this group of species have been bred to to be super aggressive through artificial selection, to kill a rival male for the pleasure of the breeders and those that participate in animal cruelty and gambling !

I have contacted the US Fish & Wildlife Service and the corporate stores to find out if these breeders are reintroducing these GMOs back into the wild. If they are I will never buy another Betta because buying one supports gambling and animal cruelty in 3rd world countries where this horrific practice is allowed.

However, these are great pets and once purchased should be taken care of properly thank you so much for finding these wonderful heaters for the Betta hybrids that they sell at pet stores. I love all 20 of mine and I have to purchase 5 heaters for 5 five gallon tanks. Finally thank you KDeus for your effort in finding perfect inexpensive heaters for Betta fishes, keep in mind what you have purchased and why it was bred, and enjoy your Betta. These poor fishes took tens of thousands of years to evolve into what they are, with some selective breeding, we as consumers cannot buy a Betta that is a wild type native fish, that is perfectly adapted to the environment where it resides. Instead we get GMOs that if released back into the wild in its native range would back cross with the ancestral strains, and cause an ecological disaster. All of you should research these animals on Google and take a look at the beautiful native species NOT hybrids. See what natural selection has produced and compare it to these artificially bred GMOs. It is worth it, and when you see these native fishes you will realize there has never been a need to alter their genetic make up, they are the most beautiful animals that have adapted to the low oxygen (anoxic) conditions so well. The wild native fishes in the genus Betta are some of the most wonderful animals in the tropics.

Oglesby 17 months ago

Thank you so much for this helpful information. I have been doing research on the best heater for my new fluval spec 5 gallon setup (for my future betta) and you helped me tremendously in making my decision.

janise cross 7 months ago

from…oh my gosh. thank you SO much for doing the research for me! my husband had progressive ms and his caregiver nurse bought him this betta fish which we came to love. I have taught him to follow my finger and eat out of my hand and we are working on jumping through a hoop. He named him Number One (Kinda Jokingly they both thought he would only last a matter or weeks…) I kind of renamed him Mr. Wonderful. Sometimes I call him Mister FancyPants. She brought it in a 1 gallon cube. Within a month I realized how horribly disgusting that water was getting and upgraded him to a 2.5 gallon with many new goodies in there. Im so SAD my husband passed away January 18. Mr. Wonderful hasn't seemed quite right. I upgraded him again to a 5 galling with even more new goodies. He seemed happier but still not at all himself. THen upon more research I realized that is water temp because of it being winter is only hovering at the low end of the green scale on the thermometer and he's too cold!! So I panicked and was going to run out and just buy any little heater at Petco. Then I saw your post so I am ordering this right now with next day delivery!! Mr Wonderful needs a Heater!!

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