Hydor Theo 25 Watt Heater Review

Hydor Theo 25 Watt Heater Review – Here Are Our Thoughts

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The 25-watt Hydor Theo Aquarium Heater is a popular heater for betta tanks because its small size and low wattage make it ideal for a smaller aquarium.

Since bettas are small fish that are often kept in tanks of just 5 or 7 gallons, many fish tank heaters will be too big or powerful to fit the tank properly.

This heater has been made with the nano tank in mind and is designed for aquariums between 2-7 gallons.

It is a reliable and consistent heater that comes with some neat features and some innovative advancements.

Let’s take a more in-depth look at what makes it a great choice for betta tanks!


Hydor Theo Shatterproof Aquarium Heater, 25-Watt & Ideal For 2 To 7 Gallon Fish Tanks, 7-Inches Long
  • Hydor Theo 25-Watt Aquarium Heater is a fully submersible; shatter-proof glass aquarium heater ideal for 2-to-7-gallon fish tanks; measures 7 inches long
  • Maximum Safety Aquarium Heater; THEO fish tank heater is shock resistant thanks to the patented PTC heating element and the shatterproof glass tube
  • Free positioning in aquarium with supplied suction cups; The Theo aquarium heater can be placed in the fish tank vertically, horizontally and fully submerged

Although this aquarium heater may cost just a bit more than the average betta tank heater, it’s definitely one of those “you get what you pay for” purchases that I highly recommend you take a close look at.

When it comes to aquarium heaters, I ultimately want to be able to forget about them as soon as I set them up.

To me, not thinking about a heater once it’s already installed translates into added peace of mind – one less thing I have to worry about. This heater is reliable. It does its job, and it does it consistently well.

Spending any given amount of time in the betta fish hobby, a person will quickly learn that the betta fish, along with their smaller size aquariums, can be a relatively difficult thing to heat properly.

Smaller-sized tanks of water generally do not heat accurately or safely. Choosing the right heater for the job is more than important in making sure that your betta fish lives a happy and healthy life without the threat of becoming too cold or accidentally cooked!

This 25-watt heater will be an amazing addition to any betta fish aquarium from 2.5 to 5 gallons.


Overall, the Hydor Theo Aquarium Heater packs a pretty big punch in a really small package.

Seeing as you yourself can’t physically pick up this aquarium heater and get a better feel for it in person, I want to include a dimension section to give you a more detailed idea of how it will fit your own aquarium needs.

The following dimensions may be off by just a hair seeing as I’m not the best in the world with a measuring tape. But, if the numbers are off, they will only be off by a fraction of an inch.

The total length is 7 inches. This is measured from the very tip of the controller on top to the very end of the glass heating element on the bottom.

This aquarium heater at its widest measurement is just under 1.5 inches in diameter. If you take a look at the picture, this would be at the larger control end – the part of the device that houses the temperature selection dial.

The glass heating element area measures just under an inch in diameter.

Once mounted to the wall of the aquarium with the use of the included suction cups, the distance from the aquarium wall to the farthest protruding point on the heater is 1.5 inches.

The “Theo” Heater In Action

Placing it in your aquarium vertically, and allowing some wiggle room around it both to the sides and to the top and bottom, the 25 Watt Hydor Theo aquarium heater will keep your 5-gallon betta tank consistently and reliably warm and cozy.

The easily identifiable thermostat located on its top is uniquely labeled in degrees of temperature rather than the commonly found and overly simplistic + and – that most aquarium heaters with built-in thermostats provide.

One tiny little detail that you should know about before you buy this, so you do not get surprised later on, is that, right out of the box, the true water temperature does not always match the temperature selected on the dial.

That is to say, you need to be a little forgiving with this heater as it may be off by plus or minus a degree or two in either direction.

Hydor Theo Shatterproof Aquarium Heater, 25-Watt & Ideal For 2 To 7 Gallon Fish Tanks, 7-Inches Long
  • Hydor Theo 25-Watt Aquarium Heater is a fully submersible; shatter-proof glass aquarium heater ideal for 2-to-7-gallon fish tanks; measures 7 inches long
  • Maximum Safety Aquarium Heater; THEO fish tank heater is shock resistant thanks to the patented PTC heating element and the shatterproof glass tube
  • Free positioning in aquarium with supplied suction cups; The Theo aquarium heater can be placed in the fish tank vertically, horizontally and fully submerged

Once you know what the temperature variance is though, you can make a mental note of it and dial in the perfect temperature every single time.

To further explain, let’s say that you had the temperature dial set at 78°F and found that the true temperature of the aquarium (using a thermometer) was 77°F.

All you would have to do is simply turn the dial to 79°F to reach that 78°F perfect temperature. Easy-peasy.

This heater is extremely reliable for your tank and will remove any worries you will have about whether your heater is actually working or not, therefore having the heater measurement off by a degree or so is a very small discrepancy.

Especially considering the fact that all you have to do is turn the dial up or down a notch. We’re sure that you will love this heater.

25 watts for an aquarium heater is generally considered a very low-wattage product. However, for betta fish and the typical-size aquariums that they are housed in, this is more than likely the perfect wattage for your needs.

One thing you have to take into consideration is the ambient temperature of your own house. You also need to understand that you want to heat your betta fish tank to about 78 to 80°F.

It takes a good amount of trial and error, and often years of experience, to understand what wattage will be required to heat a given volume of water.

But, to try and make things easy for you, let’s say that you have a 5-gallon aquarium. If your personal room is often consistently heated to about 68 to 70°F, this heater will allow you to heat that 5-gallon aquarium to the required 78 to 80°F.

However, let’s say that your house (or your room) is sitting at about 65°F. This 25 W heater will just barely be able to reach that perfect 78°F mark.

If you did the math you’ll notice that that’s a 13° difference between the 65°F of your room and the 78°F mark that you want your 5-gallon aquarium to be at.

Putting it another way, if your house gets colder than 65°F, you may want to get a higher wattage heater than 25 W to heat a 5-gallon aquarium. If you were to use this in anything less than a 5-gallon aquarium, this heater will be more than enough.

People that are new to aquarium heaters with built-in thermostat temperature selection dials often run into the same predicaments, and end up asking the same sort of questions while trying to figure everything out.

These are not very difficult devices to use, and once you have time to play with it, they will become as easy to use as the light switch in your room.

However, one of the most common comments about these thermostat heaters is that people often don’t understand why their aquarium won’t reach the desired temperature even at a “maximum” setting.

Nine out of 10 times the people running into this sort of heating problem have this heater installed in a betta tank that is too large.

For example, let’s look at this 25-watt heater installed in a 10-gallon aquarium.

Even though the temperature dial is set all the way at 80°, depending on the volume of water in the betta tank in the ambient temperature of the surrounding room, just because you set the thermostat dial to 80°F doesn’t necessarily mean it will actually reach that temperature.

Hopefully, that clarifies the situation and the predicament for you.

So, with this particular 25-watt heater, stick to aquariums no larger than five, maybe 5.5 gallons.

Hydor Heating, So Hot Right Now…

One other really neat feature about this Hydor submersible glass betta tank heater is that once the desired temperatures are reached, the device will turn off.

Equally as interesting is the fact that this heater will alternate between on and off states while it is heating. The purpose of that is so that the heating element itself will not malfunction due to being constantly “on”.

Say, for instance, that it is particularly cold outside during winter at your house, and you have this little heater trying its best to maintain the maximum setting in order to keep your betta fish comfortable (during the winter, probably more comfortable than you are!).

As it’s trying its hardest to keep that temperature setting, this heater will fluctuate between on and off so as not to cause any damage to the heating unit itself, and this will actually prolong its life so you don’t have to buy another heater sooner than you’d like.

It’s a pretty smart aquarium heater!

It’s worth reiterating that this is a good heater for you if you have a 5-gallon aquarium and your house is consistently no colder than the 69°F and 70°F range.

If your house is anything colder than that, you might want to bump up the wattage on the aquarium heater that you purchase.

If your aquarium is anything larger than that, you might think about purchasing a higher-wattage aquarium heater. Hydor makes a wide array of “Theo” aquarium heaters, all with different wattages. So, don’t worry, you don’t really have to look too far.

One thing you should pay special attention to when setting up this aquarium heater in your betta tank is that this heater has a water fill line indication on it.

You must submerge this aquarium heater at least up to the line for the heater to work properly. If you don’t submerge this heater up to that water indication line, the heater may malfunction, and it will also not heat your aquarium accurately.

There are certain parts of this aquarium heater that are not supposed to be exposed to air during operation. That’s the reason for this water line indicator.

Hydor Theo Shatterproof Aquarium Heater, 25-Watt & Ideal For 2 To 7 Gallon Fish Tanks, 7-Inches Long
  • Hydor Theo 25-Watt Aquarium Heater is a fully submersible; shatter-proof glass aquarium heater ideal for 2-to-7-gallon fish tanks; measures 7 inches long
  • Maximum Safety Aquarium Heater; THEO fish tank heater is shock resistant thanks to the patented PTC heating element and the shatterproof glass tube
  • Free positioning in aquarium with supplied suction cups; The Theo aquarium heater can be placed in the fish tank vertically, horizontally and fully submerged


Can I Place Silk Plants in Front of This Heater?

In short – yes, you can.

You wouldn’t want to place the silk plants close enough that they would be in contact with the heating element, though.

In general, it’s also a good idea to place this heater nearer to the output flow of your aquarium filtration system. Doing so will help evenly distribute the warm water surrounding the aquarium heater.

How Long Is the Power Cord on This Heater?

Depending on where you measure to and from, about 72 to 73 inches in total.

How Quickly Does This Aquarium Heater Raise the Temperature of a Betta Tank?

That depends almost entirely on the capacity of your aquarium.

The larger the aquarium, the longer it will take to heat thoroughly. The smaller the aquarium, the more rapidly the temperature will increase.

Does This Heater Have Celsius Markings as Well as Fahrenheit Markings on the Dial?

No, it does not. The temperature indication markings are all in Fahrenheit.

Seeing as I personally use both from time to time, a fine-tipped permanent marker can work wonders for adding your own numbers to it.

Can I Use This Heater in a 1.5-Gallon Tank?

I personally would advise against it.

For this 25 W heater, I wouldn’t place it in anything less than a 2.5-gallon aquarium. Anything smaller would be overkill.

In my experience, this heater would work best and an aquarium with a 2.5 to 5-gallon capacity.

What Is the Maximum Temperature Setting on This Aquarium Heater?

The highest temperature number on the dial reads 87°F.

What Is the Lowest Temperature Setting on This Aquarium Heater?

The lowest temperature number on the dial reads 67°F.

How Consistent Is the Temperature When Using This Betta Tank Heater?

That really depends on the ambient temperature of the room your aquarium is in and the capacity of the aquarium itself.

This 25 W heater will consistently heat a 3-gallon aquarium while the ambient temperature outside of it is in the mid 60°F range. A 50 W heater would do the same for a 5-gallon aquarium.

One thing I will say about this Hydor Theo aquarium heater is that it is incredibly reliable. Consistency is off the charts when set up properly – meaning it delivers consistently perfect temperatures.

Will This Aquarium Heater Work in a 10-Gallon Aquarium?

Again, that depends on the wattage of the model you choose.

The 25 W heater is definitely not enough to heat a 10-gallon aquarium. A 50 W model would be enough if the ambient room temperature your aquarium is in stays a consistent 70°F.

Can I Use This Heater in a Plastic or Acrylic Aquarium?

Absolutely! Hydor has designed this aquarium heater for use in all sorts of aquariums. Glass acrylic and plastic are all safe, meaning that nothing will melt.

Are You Supposed to Fully Submerge This Heater?

Yes, you are.

Fully submerged means including the red dial on top. All of it goes underneath the water line.

The only thing that should be peeking out of the top of the water line is the power cord that leads to the wall outlet.

How Can I Tell if the Heater Is on or Off?

The Hydor Theo aquarium heater has a built-in small orange light that indicates whether it is currently heating your aquarium.

When the small orange light is off, the device is off, or rather it is not currently heating. When the indicator light is on, it’s in work mode and heating your tank.

The small orange light is built into the middle of the glass part housing the heating element.

The orange light is not very bright and not intrusive at all, it’s just bright enough so you can glance at it to confirm that it’s working.

Can I Place This Aquarium Heater Underneath the Substrate?

No, you cannot.

This device is made to be affixed to the side wall of the inside of your aquarium. Placing it in contact with the substrate or other aquarium accessories could eventually lead to a device malfunction.

That is something you do not want inside your betta aquarium. While there are devices on the market that are meant to be placed underneath the substrate, this is not one of them.

Will This Aquarium Heater Be Good for Beginners?

This is one of the easiest-to-use aquarium heaters around today, and I would definitely recommend this to the new betta owner.

Remember, however, that 25 W is for up to a 5-gallon tank and 50 W is for over that 5-gallon limit.

Will My Fish Get Hurt if They Touch This Aquarium Heater?

That’s actually an incredibly common question.

The answer is – no, they will not get hurt. These aquarium heaters are made to be safe for all the inhabitants living inside your aquarium.

The housing on the outside of the heating element is specially designed to be safe for accidental “bump-ins” by fish. You’re not putting a lightsaber into your live aquarium!

Can I Use This Aquarium Heater in a 15-Gallon Tank?

This brand? Yes. This wattage? No.

You should think about getting a 100 W heater for anything larger than 15 gallons.

The Hydor Theo does in fact come in 100 W. The funny thing about that is that it’s actually cheaper than the lower-wattage versions.

Will This Aquarium Heater Work in a 5-gallon Tank With an Ambient Room Temperature of 77°F?

Yes, it will.

You’ll want to start out at the lowest setting, though. And, when you turn it on, you’ll want to keep a close eye on it.

77°F is already very close to the very perfect temperature the betta fish want. 70°F to 80°F is the perfect temperature range.

Turning this heater onto the lowest setting and letting it sit for an hour while constantly keeping an eye on it would be my recommended course of action.

You do not want to cook your betta fish. But, good on you for trying to achieve that one-degree difference!

I Have a 5-gallon Tank and House Ranges From 61°F to 92°F. What Wattage Should I Get?

The 50 W version of this Hydor Theo aquarium heater is the one that you want.

It will allow for better control during the highs and lows that your house seems to get. The 25 W heater would not be adequate for the low 60° temperatures throughout your home.

This probably shouldn’t need to be said as it’s common sense, but you shouldn’t have the heater turned on when it’s 92° in your house.

The Orange Indicator Light Turned Off. Is It Still Heating My Tank?

The Theo tank heater is monitoring the heat of your tank. It is not actively in the “on” setting, but it’s making sure that the temperatures are still within range.

When the orange pilot light turns on, the heating element will start warming up.

When your pilot light is off, and the heater is plugged into the wall, the heater is monitoring the temperature of your betta aquarium.

Can I Use This Aquarium Heater in a 1-gallon Betta Tank?

Please do not do that!

1 gallon is not a large enough area for your fish to be happy in, and, on top of that, this heater would be too much for that small tank.

You would probably end up causing harm to your fish.

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