Interaction between pH, EC, and Redox sensors?

When your Arduino locks every other day, I'd expect one of these two:

- a memory leak or invalid address used somewhere in the code, that keeps adding up until full, or overwrites something

- a power glitch in the utility line that's so big that it corrupts the arduino.

We have power outages occaissionally, but I thought it maybe wont be a problem, because the system would just restart after the outage...

That rules out suspect #2 then.

possible issue #3: an overrun of a variable or timer that's unhandled

LukasB said:

We suspect that the sensors (which are constantly submerged in water next to each other) influence each other and cause this shut down.

What diagnostic steps have you taken to come to that conclusion ?

Have you tried running the system with only one and two sensor(s) active at a time ?

What makes you think that it is a sensor issue rather than a programming issue ?

When you say 'shut down' what do you mean ? Is the system programmed to shut down given a specific condition ? or are you perhaps assuming that the Arduino program has stopped running because it isn't outputting anything visible ? e.g. it may still be running but just in an infinite loop that doesn't output anything.

The scenario you describe doesn't sound like sensor interaction. You could implement a watchdog reset algorithm until you have discovered the root cause.

Check all wait loops for for possible endless loop conditions - waiting for something that never happens.

You might want to share a (very detailed) block diagram, if not a complete schematic (you could gray-out anything you're sure is irrelevant if you don't wish to share it all), and photos of the setup.

Often things can be spotted from the circuit, or visually from the set-up, that might be causing "unexpected" things to occur that might not be currently being handled by the code. The block diagram or description will indicate which serial interfaces you're using, or which signals are analog, and the power source(s) for the various components. 

can you run on battery to eliminate power outage 

and the sensors out of the liquid to eliminate water as a cause