Redunant pH decrease logic

Hi,

I have a planted tank in which I use CO2. The profilux 4 works as expected, controlling my PH when I have the lights on. I use the Nocturnal change of the PH to have a change of 1 degree when my lights are off.

My problem: I have one CO2 cylinder and when it gets empty, my PH climbs and I get algae. This normally takes a day for me to notice that the cylinder is empty (it will depend at which time of day this happens).

I want to know if it is possible to have some kind of alarm when the PH has not come down in X time.
For example, if CO2 does not go down when it is expected, I would trigger an alarm which I can then use to stop the current switch (for cylinder 1) and activate another switch (for cylinder 2).

With the current Nocturnal change, I would get the alarm as soon as the change is made. I did not try to use the Dynamic Nominal Value, would this be the only way to achieve this?