Automatic aquarium controller

I would recommend two of each sensor. The sensors should be different part numbers to avoid a common failure and recommend different communications on each. If one fails there will be a difference and you can set off an alarm that something is amiss. They need to match within a tolerance that you will need to determine, but much safer that way. Also perform a 'what-if' analysis. This is nothing more than stating, if a sensor fails, what will the result be for the system? The goal being to list every possible failure mode ane how the system should respond. You can adjust the system response to the 'safest' path based on the 'what-if' analysis. Be sure to test every failure mode by emulating it, to verify the software reacts correctly and performs the proper action based on the failure. This will protect your precious fish.