your talking about a backup system in case of failure..I have been hearing and reading about more and more heater malfunctions and am really starting to consider hooking up thermostats to all my heaters. I'm kinda unsure about which work well and whether they work with all heaters. I really don't want to spend over $50-$75 per tank and was hoping to find something that was cheap, reliable, and easy for all my tanks.
Plenty on the worlds marketplace for from $10-$35...
AC 110V 10A LED Digital Intelligent Pre-wired Temperature Controller Thermostat
$16.59.. Just set it to go off a few degrees past current tank heater set point...