pH can change for a number of reasons, but CO2 induce pH changes play no role, I repeat and I will likely repeat this 10,000 more times, no role in providing a "better" stable place for any fish, let alone discus.
Now changing the salt content and raising the KH up rapidly is something totally different and can kill fish.
But pH has a number of things that can cause it to move,.
A pH controller offers nothig better for your fish vs on durin gthe day and off at night.
I'd argue that leaving high levels of CO2 on 24/7 and chronically high CO2 is much worst than the pH effects from not adding it at night.
Fish do not need CO2 at night, nor do bacteria, nor plants, so why add it?
The only good reason I've heard is: it's simpler for folks that add CO2 into their filters.
But using a solenoid/air stone works, or a powerhead that are connected to the light timer.
That's simple to do and it allows you to add more CO2 when you need it and then only when the plants are also adding higher levels of O2.
If you honestly believe that the pH changes causes fish stress/worst etc, then you better never do water changes either in a CO2 enriched tank...........without adding the same amount of CO2 directly to the replacement water, which no one ever does.............
We do 50-75% even water changes in CO2 planted tanks with plain old tap without issues and folks have for the last 20 years of CO2 injection usage.
Never bothered fish before. The tap's pH is dramatically different than a CO2 enriched tank. I've bred Discus in my planted tanks in the past. They did not care about pH either.