Review on IOAOI 50W Aquarium Heater for up to 20 Gallon Tanks - Shatterproof, Suitable for Marine and Fresh Water - Engineering Plastic Tube & Stainless Steel Inside - Adjustable Temperature by Janet Bartling

I installed this heater in my 10 gallon aquarium where I race without fish. Let it sit for an hour before turning it on and rotating it 80 degrees. The temperature has never risen above 77. I keep turning the temperature higher and higher. The pace stayed at 77.6. Return process started. As I removed the heater from the tank, I took a close look at the temperature controller. The problem encountered was that the rubber grommet on the top of the heater that the handle protruded from was preventing the handle from moving. I was able to move the rubber a bit and rotate the handle 80 degrees. I put it back in my tank, waited a while, plugged it in and now it keeps the temp at 80 degrees. The seller should solve this problem to avoid unnecessary returns. You think you are turning the knob, but the gasket prevents movement.