Hydor 25W Submersible Aquarium Heater - Original Theo

I was only hesitant to purchase this because I needed to use it for a ten gallon tank (It's designed for a 7gal) and this was the most affordable option. I urge you to not be anxious yourself, however, as this heater is splendid. It looks reasonably nice and will heat your water (even in a ten-gallon tank); what more can I say? If you need an aquarium heater and have a 10gal, don't sweat it. Just buy it.

Also, it's worth mentioning that the heater doesn't look exactly like the one in the main picture. It's shorter and fatter (proportionately at least...) than pictured. This isn't a bad thing. Just don't freak out, because you ARE getting what you ordered.

This is a nice choice for a tank that holds up to 7 gallons. It's totally submersible, ACCURATE, has a very effective thermostat, and is fairly unobtrusive. One feature I like is that its light glows while the water is heating and shuts off once the correct temperature has been reached. It's a good fit for a tank such as the Eclipse 3, especially if you want to house a betta (it also works well in an Eclipse 6); the Theo allows me to acclimate new fish at 86º for a week or so to rid them of any ich they might be harboring, after which it's easy to gradually reduce the temperature to my (and their) preferred 82º.

I also use Marineland Stealths, which I like because they're plastic but frankly, it bears mentioning that Marineland's "lifetime guarantee" carries an Orwellian caveat, i.e., some Stealths are more guaranteed than others. Rather than providing mail-in product registration to activate the warranty, you're expected to hang on to your receipt...well...for a lifetime (yeah, right). In addition, if they don't like the vendor, e.g., you bought it brand new and sealed but from an eBay or Amazon seller, you can kiss your hopes of replacement for a defective unit goodbye (they claim this is due to widespread fraud and people trying to obtain replacements for units found in the trash, but why were so many units trashed if they work forever, and who are these dementos who make a career of scavenging for them? Liver-lipped Finnish Ugly Boy trolls?). Marineland (actually, Aquarium Systems; they manufacture the item in Italy), after much coaxing, replaced a brand-new dead unit for me, but it's only carrying a 90-day warranty; this is because I bought it on eBay and I received an e-slap to the effect that NONE of my other Internet purchases would be covered. On this basis, my suggestion is to boycott Stealths (unless you want to pay list at a pet store) and go with a Theo. They're also manufactured in Italy, they do a great job, and I suspect they're nicer about warranty issues.

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I bought a Marineland 10W heaters for my 2.5 gallon tank, but found that it overheated the water way too much. That heater may have a sensor that tells it when the water temp is too cold, but it does not shut off when the water temp skyrockets above 78 degrees Fahrenheit.

This heater, on the other hand, while larger and more obtrusive, does a much better job in regulating water temperature. I never have to worry about finding my tank temps in the high 80s the way I did with the other heater. Also, the fact that it has a regulator dial is a huge plus. While the dial calibration may be off by a degree or two, it does work. You turn it up, the water temp goes up (but not too high). Also, the little indicator light is a nice way of checking that the heater is on and working.

Even for small tanks, I would skip the unregulated 7.5W and 10W heaters and go with this one.

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I have owned two Hydor Theo 25W heaters. The first I used in a 2.5gal aquarium with betta for 5 months, and it worked great. I got rid of the 2.5 gal tank for a 5.5gal tank, and the heater worked for only a few more months. At first, the temperature was 2 degrees higher then what I set it to. Soon, it was 4 degrees higher. Finally, it consistently kept it at over 8 degrees higher, and at this point the LED/heater never shut off. Soon it stopped heating altogether. I bought a second one which malfunctioned within three months of use in the 5.5 gal tank. My betta got developed severe fin rot from the inconsistent temperature.

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I purchased this for an Eclipse Hex 5 tank and it works perfectly. Heater is very accurate I have it set at 78 degrees, and my thermometer verifies that the water is actually 78 degrees. There is an orange light inside the glass tube near the top that glows to let you know the heater is on, and when the water reaches the set temperature the light turns off to indicate that the heater is off. The heater is very high quality and feels very durable. The package also includes two suction cup mounts.

Like a previous commenter noted, the actual heater is shorter / stubbier than the one pictured. It worked out great for me since the Hex 5 tank isn't that tall.

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