I can't understand why Eheim heaters have such a good reputation. I have three, and none of them are perfect for their price. Generally, I believe they struggle with temperature regulation. Specifically, this one maintains the temperature just fine, but to adjust it, you have to make small tweaks until you get it right. I wanted 25 degrees, and I have it somewhere around 24. And this changes every time I have to unplug it. Also, I want to emphasize that it's not just mine that has a deviation like someone might say 1 degree. I mean, if I want 24, I have to set it to 23 or 26 to get 25. If you adjust it every time, this can change. It also matters whether you adjust it by increasing or decreasing the temperature. For example, if I want 25, I set it to 22 and then see after a while what temperature the aquarium has. I increase it very slightly without looking at the display and observe the aquarium's temperature. I repeat the whole process with fine-tuning until it locks in at the desired 25. If a mistake happens and I raise it too much and it exceeds 25 and I need to lower it, then everything changes. For instance, if during the increase to 23.5, it registers 24.6 and I set it to 24 and it registers 25.5, if I lower it back to 23.5, it won't register 24.6 but might register 25.2. In general, adjusting it for a stable and desired temperature is incredibly difficult. I use the above as an example and not that I am looking for and striving for such absolute temperature. I want to quickly and easily set a temperature and get approximately the desired one, in my case around 25. Not to set 24 and have it register 26-26.5, and with a slight turn, it's 25, and for that point to change and depend on the turn for decreasing or increasing the temperature. I emphasize again that I have 3 that do this, but 2 are the Jager, where things are worse.

I declare myself completely satisfied with the product, having lived with it for some time now.

My previous heater was also from Eheim, for several years, but it was from the Jaeger series with manual calibration. Based on reviews, I decided to switch to the electronically controlled one, also considering the price, as the difference from the manual one was very small.

The Thermocontrol E came pre-set at 26 degrees, I simply installed it and when the aquarium reached a stable temperature, I checked it with a glass thermometer and it was exactly 26 degrees. I have checked it several times since then and the reading is always the same. So everything is good. Also, I no longer have the insecurity of accidentally moving the orange ring of the Jaeger and ending up setting a different temperature than I want, essentially losing the true knowledge of whether the device is properly regulating the temperature and has suffered any damage.

I really like the dual-color LED indicator that changes from green to red depending on whether it is heating or not, so I know exactly if it is functioning or not (something I couldn't know with the Jaeger) and if it is heating.

The double suction cup is great, which is not surprising since the company has been producing quality products for many years.

I still have to see its lifespan. The fact that it is a product from the same manufacturer is a guarantee but also a heavy legacy. Jaeger heaters are very sturdy constructions and the bar is set very high. I will update in due course.
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Update 11/02/24
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Just today I realized that the heater is not working. So we are talking about not even 15 months.

Therefore, I am deducting one star. I am not deducting a second one (even though the lifespan was very short) because the company provides a 3-year warranty for the device, which is a significant advantage for me.

It should be noted that the aquarium operates behind a 2kVA OnLine APC UPS. So we can remove the "power quality" factor from the equation.