I did not read this whole tread, but I wanted to add that I also had a Marineland Stealth VisiTherm Heaters fail at almost exactly 12 months. I asked PetCo about it, they said I needed to take it up with manufacturer, which I never did. I replaced it with a Pro model, which is going fine for 4 months. It's my own fault it never contacted Marineland, but it's interesting there are so many reported issues.
My own symptoms were as follows:
(1) Heater stopped working, noticed the thermometer was reading 65 degrees one day in DEC, instead of normal 79 F. I stuck my hand in, and it was freezing.
(2) I turned the knob up and down and felt the heater come back on while heater was underwater by lightly clutching in my palm. Water was so cold I could definitely tell it kicked on, even though these heaters never get 'hot' to the touch in my experience.
(3) Assumed all was well, and checked it after an hour or two, and it was back to around 79F. I left it alone again, assuming all was well. I think I set it at 81 to get it to 79.
(4) Next day, checked again, this time it was like 85F! Unplugged and pulled heater out of tank. This thing was all over the map, and unable to either tell when it has come up to temp, or when it was time to turn on. I think I was pushing it too hard with Maine winters, and having to keep tank 20 degree F above room temp. Went to Petco and bought a Pro model, which in hindsight should probably have been a different brand.
(5) Never followed up because I didn’t realize there are many known issues with this heater. But here I am now adding a voice to the discussion in hopes they figure out what the hell is going on.
P.s. NO fish died as a result of going from 65-85 in the course of 24 hours. Or at least nothing died for another week+.