Hydor Theo Shatterproof Aquarium Heater

By Hydor
Rated 3.4062 out of 5 stars


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$22.99Chewy Price
$38.95Chewy Price

In Stock
FREE delivery over $35
Arrives between Sep 26 and Oct 04

About This Item


  • A fully submersible, shatter-proof glass aquarium heater.
  • This heater is shock resistant thanks to the patented PTC heating element and the shatterproof glass tube.
  • The supplied suction cups allow you to position the heater anywhere in your aquarium. You can place it in the fish tank vertically, horizontally and fully submerged.
  • If heater senses abnormal functioning, the patented PTC heating system guarantees completely safe operation in case of dry running.
  • This universal aquarium heater may be used in freshwater or marine water fish tanks.

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Fish TypeMarine Fish, Marine Invertebrate, Tropical FreshwaterBetta, Cichlid, Goldfish, Marine Fish, Tropical FreshwaterMarine Fish, Tropical FreshwaterMarine Fish, Tropical FreshwaterMarine Invertebrate, Tropical Freshwater
Aquarium TypeSaltwater, FreshwaterSaltwater, FreshwaterFreshwaterFreshwaterSaltwater, Freshwater
MaterialPlasticMetal, Stainless SteelPlastic, PolypropylenePlastic

Questions & Answers

7 Customer Questions

What is the cord length?
Answer by ChewyNov 03, 2017
The cord is 6 feet long. The majority of Hydor's products have a standard 6-foot cord.

Can the temperature be adjusted?
Answer by ChewyOct 20, 2017
The temperature can be adjusted.

What neater is best for a 5 gallon tank?
Answer by ChewyAug 25, 2018
The 25 Watt Aquarium Heater is ideal for 2 - 7 gallon aquariums.

Can this thermometer be used in a turtle aquarium?
Answer by ChewyAug 26, 2018
This heater is suitable in saltwater and freshwater tanks.

ReviewsWrite a Review

Rated 3.4062 out of 5 stars
32 Ratings
Would recommend to a friend

32 Customer Reviews

Rated 2 out of 5 stars
By MissB on Oct 10, 2022

Be Cautious

Ordered because I wanted a heater for my 5 gallon tank that I could adjust. I didn’t read all the reviews. My experience was similar to most of theirs. I set the heater to 79 and went to bed. Next morning I found it had heated to 84. First, I adjusted it to 75, but it still wanted to turn on. So, I unplugged the heater and waited several hours for the water to cool down to 79. Then, I turned the heater setting to 68 and plugged it back in. Over the course of 2 hours, I turned up the heater setting a degree or two until I knew it was keeping the water a consistent 79. That setting, for me, was 75. For looks, I love it. It’s slim and modern. Price was great. But, just be aware of it’s possible issues and keep an eye on it until you’re confident it’s working properly.

Rated 2 out of 5 stars
By Kat5 on Dec 31, 2018

Stuck on one Temp

This aquarium heater worked great for a few months. The design and appearance are great, but after a while I noticed that even if I had it on its lowest setting (68) it kept my tank was stuck at 82 degrees F... didn't hurt the fish at all, but had I owned fish that enjoyed a slightly cooler temp, I would've been in trouble.

Rated 1 out of 5 stars
By Mike on Dec 28, 2018

waste of money

My experience with this product was very DISAPPOINTED after buy 4 units "1 per aquarium" fully submerged in less then 2 months 3 units just burn . I call the manufacturer and they have a only 30 days guaranty. And the last unit is a PAIN,,NEVER KEEPS THE RIGHT TEMPERATURE,,NOT A RELIABLE BRAND!!!!!

Rated 5 out of 5 stars
By Jennyw on Dec 14, 2018
Top 50 Contributor

I don't trust other heaters

Sad they seem to be phasing them off of Chewy. They are all I use and so far they always seem to maintain the correct temps.

Rated 5 out of 5 stars
By AuntieM on Oct 22, 2018

Works perfectly

Keeps my betta’s 5-gallon Fluval tank at 78 degrees without the need for adjustment and fits inside the back compartment with the filter motor.

Rated 1 out of 5 stars
By danm21 on Aug 18, 2018

Bad quality

Went through 2 in about a year, they power on but around 6 months you will see burnt spots on heater glass and it won't heat the water anymore.

Rated 5 out of 5 stars
By Jemm on Apr 24, 2018

Perfect for bettas

It’s a great heater for small tanks, and the best part is that it’s adjustable which is hard to find in small heaters.

Rated 2 out of 5 stars
By Brewcity on Apr 14, 2018

Stopped Working

Got two of these the other day to replace the old ones I had(different brands) and suddenly they both stopped working. Both are in separate tanks and both tanks have dropped back down in temps...

Rated 1 out of 5 stars
By Dadams on Mar 22, 2018

Very Wrong

I had heater turned as low as possible and it still registers 86 degrees. 16 degree difference. If you love your fish please don't buy this heater. I switched to the tetra which has never let me down.

Rated 5 out of 5 stars
By Bluestraycat on Feb 17, 2018
Top 10 Contributor

Aquarium heater

I bought two of the 25 watt heaters for my 3.5 gallon Betta tanks. It's difficult to find a good, adjustable heater for a small tank. It did take a couple days to get the water up to 80 degrees, but that's probably because I keep my home on the cool side (66-68 degrees) in the winter. I've only had the heaters a couple weeks, but I'm very happy with them. The only thing I would change is there is no indicator light to show when the heater is on.