Over the years, I've used various brands of submersible heaters - Eheim Ebo Jager, Hydor, Aqueon, Marina, and Fluval among others. This post isn't a review nor does it contain any kind of personal heater ranking, however I want to express my overall satisfaction with a particular brand/model.
I've grown quite fond of the Fluval Digital E series by Hagen. There are multiple reasons for my attraction to and satisfaction with this heater, which are listed below in no particular order.
Of course, the most important aspects of a submersible heater are its dependability and accuracy. I've owned four of the E series and, to date, I've only had a problem with one. Fortunately, it was faulty right out of the box.
The temperature adjustment lever is at the top of the heater, which makes adjusting the temperature really easy and, if you mount the heater vertically, the lever can protrude above the water line allowing you to make an adjustment and stay dry doing it. Also, the colored display allows you to quickly notice a problem with the temperature, especially in the dark.
A word of caution, however. Even though these heaters have been reliable for me, I believe in redundancy. For this reason, I keep separate thermometers on all my tanks and I would encourage you to do the same if you use this heater or any other with a built in temperature display.
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