I have always used 2 heaters in all of my tanks for the past 38 years. Yep, I'm showing my age.LOL
I never had problems with the old hang on tank glass heaters when they were they only ones out there. Worse thing that ever happened was that they stuck on off. No biggies as my tanks have always been in my living rooms.
First heater to go bad on me was a
Won Brothers Titanium heater with external thermostat. Stuck on ON, and boiled my show size fish! Water was as hot as a hot tub!
Second heater to fail on me was last spring. It leaked God only knows what through a tiny pinhole in to my 155 Bow Front reef poisoning 90% of my fish. That one was a
Visitherm Stealth Heater.
Third heater to fail was just 2 days ago in my fowlr! Another
Visitherm Stealth Heater. I just found it today in pieces. I went to take it out of my sump, and almost fried myself while looking for the cord to it.;( Lost about 30 tiny brittle star fish, lg. serpent star fish, 2 chocolate chip starfish, baby Majestic angelfish. The 5 anemones looked like they would all die, the Blue Hex palys all drooping, leathers sliming like crazy, skimmer going nuts. Hoping all turns ok now that I found the culprit, and it is out.
I now run Ebo Jagers in my reef on the RK2. And I don't trust those either........