Review on DaToo 25W Mini Aquarium Heater: Submersible Betta Small Fish Tank Heater for 3-5 Gallon Tanks - 1 Year Warranty by Judy King

fulfills its purpose, I award 4 out of 5 stars. While you can't control the exact temperature, this actually maintains the specified temperature in your tank. During the day it will be a lot, but always within the specified range. It sticks well to the wall and I personally haven't had any issues with the suction power of the heater itself. The digital thermometer would not suck at first and floated away, but after a few tries I got it to sit still. (away from any flow in the tank) overall a decent buy.