I use Home Assistant to automate and monitor my aquariums

>This is the most dangerous automation. A heater stuck in the on position can kill the fish in hours, so I've really spent a lot of time and energy to ensure I've accounted for the failure modes and edge cases that could cause the automations to fail. So far so good.

This is why you use multiple heaters - each one small enough that it cannot raise the temperature to above the danger zone if stuck on.

(Admittedly, this is harder to do on very small tanks)