Hi Gabriel,
Glad things are improving.
I'm not understanding what you mean by this:
In the morning the ph was 7.4 and in the end of photoperiod 6. So 1.4 variation.
Do you mean that when the gas first turns on the pH is 7.4?
What is the pH when the lights go on?
How many hours from gas on to light on?
It seems you have arrested the decay and if new leaves are growing, then I really would not tinker too much for at least a few more weeks.
Allow the plants some stability for a while.
It's very easy to do something stupid and to ruin the improvements you have accomplished.
You could try, for example, to turn the gas on an hour earlier before the light.
Ideally of course, we would like the minimum pH to occur at lights on, but that is very difficult to achieve, and it's a very theoretical metric.
By far, more importantly is the observed health of the plants, which you have already achieved.
Yes, you could add more Carbonate to bring the alkalinity up to 4, but why bother? It's one more thing to add all the time and one more complication.
When I see that my plants are getting healthier, I usually abandon attempts to achieve some theoretical goal.
Naturally, if their health starts to decline again, then I would seek further improvements and further tweaking, but I try to discourage people from chasing numbers instead of being satisfied with the physical improvement in the tank.
Cheers,