Jebo Aquarium Fish Tank Automatic Heaters 100w 40gal

I have one of these heaters, and for some reason it allows the tank temperature to fluctuate quite a bit. :(

This heater came preset. I didn't have to adjust anything. I am using it in a 30 gal tank. It keeps the water at just the temp I want for my tropical fish. The temperature has remained constant. I had tried other brands, but they didn't do what I wanted so I searched on Amazon and found this brand. I am totally happy. I bought more for my other tanks because I was so pleased with the accuracy and temps holding constant.

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