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Product Reviews
5493 reviews on 1118 products!

ProductsTemperature ControlHeaters
Hydor THEO
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Product Description:
Hydor THEOThe first shatter-proof heater thanks to the peculiar glass tube.

Maximum security: : THEO is shock resistant thanks to the PTC heating element and the shatterproof glass tube.

Free positioning in aquarium with supplied suction cups.

Self-limiting: : the patented PTC heating system guarantees operation in complete safety because in case of dry running there is no danger for damage.
Product Specifications:
25W - 400W sizes
User Ratings: (33 reviews)
Overall: 5.42
Value/Cost: 5.48
Effectiveness: 4.79
Quality: 4.85
Ease of use: 6.58
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Found: 33 reviews Page  1 2 3 - View all
partsrep has used this product for 3 Years or More.
Overall Rating: 5/10 March 25, 2013
"Was OK, now it's not"
I used a 100 watt on a 10G for several years. I unplugged it to do some tank moving and when I plugged it back in it overheats the tank even on the lowest setting.
theoryguru has used this product for 3-12 Months.
Overall Rating: 3.25/10 April 11, 2012
"Better than nothing"
I set the temp to 78F and after a week it suddenly jumped to 84F. I reset the temp several times and now set to its lowest setting and still remains at 84F. My fish aren't bothered with the warmer than average temp, but I would never buy a Hydor. This particular heater came with a used setup.
I'm glad I didn't buy it new, better than nothing.
the blur has used this product for 1-3 Months.
Overall Rating: 1.75/10 April 13, 2011
"failed "
failed in a few months.
Geddonight has used this product for 3-12 Months.
Overall Rating: 4.25/10 February 9, 2010
"Get what you pay for"
I've purchased many heaters for my tanks. These heaters consistently fail "on" so they overheat the tank.
Ali1 has used this product for 3-12 Months.
Overall Rating: 1/10 December 30, 2009
overcooked my fish. Garbage product, only had it for 7 months, if that. If you love fish, stay away from this heater unless u intend on cooking for supper.
DrgRcr has used this product for 3-12 Months.
Overall Rating: 4/10 November 8, 2009
"Incredibly Inaccurate"
I bought this for my 10g fry/QT tank. The temperature calibration is way off from the dial setting, and it seems any small adjustments you make cause it to either not change or change way too much. Don't waste your money.
bradp has used this product for 3-12 Months.
Overall Rating: 4.5/10 March 9, 2009
"Killed All of My Fish!"
I am sure that there are heaters that fail from all manufacturers, but I have gone through three of these heaters due to inaccuracy of +/- 5 degrees. The final heater killed about 20 Cichlids in my grow out tank when the heater failed to 94 degrees.

I will never place another one in my tanks again. Probably the worst fish keeping experience to date as the smell of a fish boil is still in my home.
edmlfc has used this product for 3-12 Months.
Overall Rating: 2.25/10 July 17, 2008
"Junk, Junk, Junk"
Don't waste your time or money on this heater. I purchsed one, glad I only purchsed the one. It's total junk! Spend a little more money and get a heater that will not fry your fish. This heater worked for maybe 3 weeks. Now the only thing I use it on is my backup water. I plug it in and put it in a 55 gallon drum of water. It takes 15-20 minutes for the water to go from 65 to 78 degrees. Just imagine what it would do to my fish if I left it in the tank for 24 hours. And I have it on the lowest setting. I will never buy another one of these. I didn't even bother to have it replaced or exchanged. I don't want to take the chance that it will fry my fish. I don't want it replaced with another piece of junk. If you have no other option watch your temp very carefully!
KillerX has used this product for 3-12 Months.
Overall Rating: 2.5/10 March 13, 2008
"If you love your fish, DONT BUY THIS HEATER!!!"
Well I posted on the forum of my experiences with these heaters. Basically long story short bought a 200watt it lasted around 3 months then it heated up for no reason took it back and got another one bought a bigger tank so I bought a 300 watts did the same thing around 2-3 months took it back and got a new one then the replaced 200 did it again! then I said ferget it im not buying anymore im keeping the 300 and thats it. Until one day I come home and my temp is around 86 degrees. Please, if you care about your fish buy a quality heater!!! If you dont care then sure save the couple of bucks and watch your fish die.
Altumdreams has used this product for 0-1 Month.
Overall Rating: 1/10 October 3, 2007
"Horrible Floating Heaters"
I purchased 3 Hydor submersible heaters and all of them float. They are so boyant that they continue to pull off the 2 suction cups attached to the heater and having it float on the surface. I've tried additional suction cups though this is rediculous. Submersible heaters shouldn't float.
75malawi has used this product for 1-3 Years.
Overall Rating: 9.25/10 September 11, 2007
"Good experiences here"
Some of these horror stories scare me. I have had nothing but good luck with my various Theo heaters (200w, 300w, 400w). Most of them are > 1 yr old, and everything is great. The temperature readout at the top is inaccurate, but that's par for the course with any heater. Just use a thermometer as well, and adjust the heater accordingly.

Hope my luck continues.
galen has used this product for 1-3 Years.
Overall Rating: 8.5/10 August 5, 2007
"Very very good heater"
I bought a 100 watt heater for my 20 gallon tank 2 years ago. I tis still working today. It keeps the temps above 70 degrees cause I set it up that way- a temp. floor as the tank neevr got above 78 degrees but in winter temps would go down to below 60 in the house. It works perfectly. Only complaint is that the paint is beginning to peel off from the water. Recommended.
spartan.mike has used this product for 0-1 Month.
Overall Rating: 4.25/10 April 29, 2007
"....good for frying fish"
I turn it on and an hour later my tank rises 6 degrees. i returned it and got another heater, and the same thing happened.
Beaglegirl has used this product for .
Overall Rating: 1/10 March 28, 2007
"Liked at first, till fish fry"
I really liked this heater at first. Kept my tank constant temp for about 6 months. Then all heck broke loose. Came home from work, digital alarm was going off,(Thank GOSH I had an alarm on this tank) my tank was at 85! AND RISING(It had been constant 78 up till then.) I turned it all the way down, but it refused to shut off! Horrible! Up all night plugging dang thing in and out trying to keep my fish alive till morning and fish store opened. STINKY HEATER almost fried my fish!
inkedskin has used this product for 1-3 Months.
Overall Rating: 9/10 March 5, 2007
"doing a fine job here."
for the price i paid, this heater is great. my 100w is only 3 months old but I've had zero trouble. its easy to use and keeps the water within about 5 degrees of the setting on the high side. it may not be the best but its a good product from my experience with it.
mohinder has used this product for 3-12 Months.
Overall Rating: 1/10 April 16, 2007
"These run about 10 degrees too high! BEWARE."
I bought three of these from Pet Solutions when I bought a new tank, and they were sold out of Ebo Jager heaters. They are compact and fairly cheap. But, all of them run about 10 degrees too high. I set them at around 78 degrees, but the water measured at 88 degrees. ALL THREE OF THE HEATERS RAN TOO HOT, AND I COULDN'T GET THEM TO RUN COOLER. Be careful, and stick with a Jager (formely Ebo Jager) heater instead.
Kuettle has used this product for 0-1 Month.
Overall Rating: 3/10 August 1, 2006
"Returned After 1 day"
Bought one for my 110G. Set the temp for 80 checked temp a few hours later had gone up to 84, turned it down to 76 and checked later temp had risen to 86. I turned it down all the way and after about 15 minutes touched the heater and practically burned my hand. Wounld not shut off. Total POS don't waste your money.
chefjimmy has used this product for 1-3 Months.
Overall Rating: 3.75/10 July 14, 2006
"i am replacing 6 now"
i purchased 6 of these heaters, right around the same time, and within 2 months 4 of them are just running on there own. the dial is set at 75 and i noticed the temp was climbing and the light was not going off. it got up to 85 degrees, and i had enough. none of them are even close to accurate.
mbunakeith has used this product for 3-12 Months.
Overall Rating: 6.25/10 May 11, 2006
"works great, when it works."
i graded it high for effectiveness because when it works, it is great. i have 3 of these (100w, 150w, 200w) and my 100w konked out on me after only 6 months. the temp would slowly go up and i would turn the heater down to compensate for it and it would be fine for a couple days. finally after about two weeks of this constant fidgeting, it just gave up on itself. it just wouldnt turn on anymore. my others are newer and seem to be working good so far. i graded low on ease of use because if want 80 degrees, you have to set it to 75 or so. dial seems to be off. it's too bad because i think the hydor inline heaters (eth300) are the best heaters ever. i went with the hydor glass style for my smaller tanks because i use HOB's on them.
mike01c has used this product for 1-3 Years.
Overall Rating: 10/10 May 5, 2006
"no problems"
I haven't had any problems with this heater. I have a 50w for my 10gallon. It's been accurate. My apt has been as cold as 58 degrees and still this maintained the water at 78 or above. I've been using it for a yr. I bought it because it was the smallest 50w on the market.
DaddyWiehe has used this product for 0-1 Month.
Overall Rating: 8.5/10 April 28, 2006
"Great deal..."
I bought two of these a few weeks ago and they are running great. Easy to adjust and to read the settings. Would recommend shopping around for best price them can be costly. My 400 watt was norm. $45 at petsmart but on thier site it was $35 so i picked up two cheaper than the rest they had to offer.
stelow has used this product for 0-1 Month.
Overall Rating: 4.5/10 April 12, 2006
"Buyer beware"
The worst heater I ever purchased. It was not working right brand new from the box. I retuned it for a refund.
dabigmandan has used this product for 1-3 Years.
Overall Rating: 1.5/10 March 15, 2006
"Terrible, worst heaters I've had"
I've had 2 Hydor Theo's. A 200watt and a 300 watt. Both Had the same problems. THEY WOULD NOT SHUT OFF! One was only a year old when it started to malfunction and the other was only 3 months old!. Do not buy this heater. Worst heaters on the market.
waterways has used this product for 1-3 Years.
Overall Rating: 10/10 March 14, 2006
"Great Heater at Good Price"
I have many large cichlid tanks and each one of them has Hydor Theo heaters. For the simple reason they keep the water temp super accurate, never a problem with it. You pretty much just set it and forget it. On top of that I have large fish that in the past have distroyed old heaters. Not these one bit. Some of them have been beaten up pretty good by some pretty big fish and not one once of damage on any of them. So a definate yes for this heater, espically if you have larger fish!
Miles43 has used this product for 1-3 Months.
Overall Rating: 9/10 March 2, 2006
"Very good heaters"
Great accuracy. Highly recomended if can get at the right price.
straitjacketstar has used this product for 1-3 Months.
Overall Rating: 9/10 March 2, 2006
"Great heaters"
Have about 5 in differn't tanks. All great. They keep the temp. w/in 1 deg. acc.
pcworks has used this product for 0-1 Month.
Overall Rating: 8.25/10 February 10, 2006
"Price was right but dial not accurate"
I have had this heater for about 3 weeks or so. IT took a little messing around with the setting to get it right. Its set at about 75 to get 79. But once set my temp does not fluctuate. So for the price I would say its ok.
rna has used this product for 1-3 Months.
Overall Rating: 7.75/10 January 25, 2006
i've searched for a small heater to put in my ac500(sw use) and this was the smallest 100w heater i could find. the first one i bought, got scared because the light would never go off. the second one i bought set it at 79F and it would heat to 85F, next one did the same, and te next did the same too.

i would just compensate for the difference, but other than that, the heater is small and compact. and it also holds the temp very well, very constant.

mixed review about this heater.
brianp has used this product for 0-1 Month.
Overall Rating: 4/10 January 24, 2006
"Unstable and unreliable"
Installed two 400 watt Theo heaters on a 150 gallon tank and set each at 79 degrees. Watched the temp jump around within a 3 degree range. Then set back to 78 and watched the temp climb to 80 degrees. Then set back to 76 and the heaters were still turning on.
brutus8 has used this product for 3-12 Months.
Overall Rating: 7.5/10 March 10, 2007
"Solid Heater"
This is a good heater for the money. The only bad thing I can say is that the water temp is a little cooler than what my heater is set on, but other than that it is pretty good.
joejoe has used this product for 3-12 Months.
Overall Rating: 8.75/10 November 14, 2005
"Great Heater"
Hydor always makes great products and this is another one of them. One nice thing about it is that it does not use a standard heat coil but seperate heating zones to disperse the heat better. Another intresting thing to prove the quality is that you can plug this thing in outside of water and it does not crack. Any other heater would. In fact you can let it get as hot as it gets and plunge it in a bucket of ice water and it will no phase it. If you can not find this heater All Glass Aquarium just released their heater and it is the same exact heater but sold under the AGA name.
Limvostov has used this product for 1-3 Months.
Overall Rating: 9.5/10 October 12, 2005
"Solid heater ... steady temperature"
I have been using this heater for about 2 months so far and it has been doing a great job on keeping the tank at a steady temperature of 78F. However, I am using the 400W heater on a 29g tank in a lab kept at 71F at all times, so I would be really upset if it could not keep the temp steady.

Being able to submerge the heater is kinda nice but since the temp setting dial is on the heater you have to get your arm wet to change it if it is fully submerged. Not really convenient in the lab.
Kinguni has used this product for 1-3 Years.
Overall Rating: 5.25/10 October 11, 2005
"Good heater, but runs too warm"
I've used 2 of these heaters, one being 175 watts and the other 100 watts and both kept my tanks warmer than I liked, even at their lowest settings. I ended up selling the 175 watt one to a fellow fish keeper who had tanks he kept warmer.

If it could keep an accurate temperature, it would be good, but keeping the tank at 82 isn't good for a lot of fish.
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