Alarms for failing sensors

Hey,

I have an automated water change set up with 4 level sensors (two top, two bottom):

• The first step is that a pump pulls water out of the aquarium up until the two sensors at the bottom switch because water is below this level.
• Then the second step consist on an electro-valve that opens to allow water from the house supply to go into the aquarium, until the two sensors at the top switch because water is above this level

I've implemented it basically using the 2 sensor ATO option (with the bottom sensors being "reversed").

I additionally incorporate salts to the aquarium on each water change, with the amount adjusted to the amount of water that I know gets changed based on the centimeters between the top and bottom sensors (and the aquarium dimensions).

The time limit for the ATO has helped me prevent issues when

• The bottom sensors do not switch when the level goes below them, so the pump keeps getting water out. Usually it takes about 7 minutes, and if it goes over 7:30, the alarm switches off and the process stops.
• The top sensors do not switch when the level goes above them, so the electrovalve keeps letting water in. Usually it takes about 27 minutes, and if it goes over 28:00, the alarm switches off and the process stops.

While that works, I have two issues:

• When the sensors fail in the opposite direction, no water gets replaced. Example: the bottom sensor is already switched as if water was below its level, even if it is not the case, or the top sensor is switched as if water was above its level, even if it is not the case. Unfortunately, I do not get a warning of the water not being changed, so I can't detect it, and on top of that, salts keep being added to the aquarium, increasing the salt levels below what fish can tolerate.
• Even when they fail in the direction described before, and the process stops, that does not prevent the added salts. Specifically, if the top sensor fails, the aquarium is not ref-filled after the water has been pumped out, but salts are added as if there was new water in the aquarium. I eventually detect it (because I see the level lower), but for some time the salts were too high.

So I wonder if there are ways to:

• Set an alarm if the sensor is in the wrong position before the process starts. (e.g. if just before the water change starts, the bottom sensor says water is below its level, then that means the sensor is failing, if just after the pumping out finishes, the top sensor says the water is above its level, that means the sensor is failing)
• Prevent salts to be added when water has not been changed for one reason or the other.

Thanks!