Aquarum Heater Review – Hydor Theo

In this review we’re going to be covering the Hydor Theo Aquarium Heaters. The Hydor Theo is a glass heater with suction cup mounts and a temperature adjustment knob on the top. They come in 25 watt, 50 watt, 100 watt, 150 watt, 200 watt, 300 watt, and 400 watt models. The tank sizes and heater length for each heater are listed at the bottom of the post.

The Hydor Theo heater is a pretty inexpensive model when compared to other heaters, so that alone is a pretty good start. They are made of shatterproof glass and have a self turn off feature if you forget to unplug it during a water change. As with most heaters the suction cup mounts are not great, ours fall off from time to time and you have to reattach them.

The adjustment knob on the top is easy to read and adjust. You turn it one click for each degree of adjustment. The settings seems to be fairly accurate as well, as long as your heater is sized right for the tank. It is a submersible heater per the manufacturer, so you can mount it wherever you’d like in your tank. The cord is 5.5′ long, which is pretty standard and adequate. The Hydor Theo also features an indicator lamp, telling you when the heater is on.

We have used 3 of the Hydor Theo 400 watt models so far. Of those 3, all of them still work but the oldest one is stuck on. Somehow the thermostat stopped working so it never shuts off, if it were in a small enough tank this could have been disastrous but because it was one of a couple heaters in a much larger tank no damage was done. It wasn’t capable of over heating the tank on its own. We still use the other two but watch them carefully. This is why a heater controller is never a bad idea, but those too can fail. I’d say overall the Hydor Theo is a pretty decent heater, especially given the low price. If you have to replace one of three every few years, that isn’t too bad if you ask me.

Pictured – Hydor Theo 400watt

Hydor Theo Heater Chart

Watts Length Aquarium Size
25 7″ 2-7 gallons
50 7″ 5-14 gallons
100 9″ 14-26 gallons
150 9″ 23-40 gallons
200 12″ 26-53 gallons
300 15-1/2″ 53-80 gallons
400 15-1/2″ 80-105 gallons

1 Comment

  • Thanks for the review; it’s interesting to hear from people who have had this model fail in the ‘on’ position. Scary stuff. I put my 50W (that is in a nano) on a temperature controller right away for this reason.

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