25W Hydor Theo Aquarium Heater Review

I really like this heater, even though it is costly for its wattage.

Small tanks, typically used for Bettas, always have the issue of heating them and due to their small size most heaters will not fit. Well, this small 25W heater works perfectly in my 5.5 gallon aquarium easily fitting vertically with plenty of space to spare above and below. The thermostat on top is labeled in temperature, versus just a + and – or arbitrary scale (like 1 to 10). The temperature may not match the dial right out of the box, but really that’s not a big deal. If you need it a degree warmer, you turn the dial up 1 degree. This is assuming you are not exceeding the capabilities of the heater, which I explain in more detail below.

5.5 Gallon Aquarium

5.5 Gallon Topfin Aquarium with a Featherfin Betta

I’d like to point out that this is a very low wattage heater. So while it works fine on my 5.5g aquarium you should know that it will only do fine if your room temperature is somewhat close to your desired tank temperature. If you keep your tank at 78 degrees, but in the winter your house is at 65 degrees this heater will not be able to make up the 13 degree difference. Small tanks loose heat fast, and this heater at just 25W will not be able to keep up with that.

Because of this some people will mistakenly say “It can’t heat my tank even when I turn it up to high”. As I mentioned earlier, these have a thermostat so they work only on temperature. If  it is not rising in temperature even after adjusting the temperature it is already doing its best to heat the tank. Turning it up to 85 degrees does not change that. The heater will not turn on anymore than it already is. You may notice it still turning off even if it is below the set temperature but this is normal as it will not stay on 24/7 to avoid super heating itself.

How low your house can be for this to work will depend greatly on your specific tank dimensions, what’s in it, and your filter flow. Because of that I can’t give you some specific number, but as a ball park in a 5 gallon aquarium I doubt you could go under 69-70 degrees in your house and maintain 78 degrees in the tank. Not saying it isn’t possible, but just advising caution.

The heater has a water fill line on it, this is a minimum fill line, which means the water must be at least up to the line for the heater to work without breaking. That line is not a maximum, you can fully submerge this heater with no issue.

In the picture below you can see the 25W Hydor Theo Aquarium Heater inside my 5.5 Gallon Aquarium along with my TOM Aquarium Mini Internal Filter.

Minifilter and Heater

TOM Aquarium Mini Internal Filter and 25W Hydor Theo Heater


  • Small Size
  • Easy to read temperature dial
  • Light indicator to show when working


  • Made of glass (risk of shattering)

Rating: 5/5

What to see more reviews? I have several more on my Reviews page.

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