That is interesting, maybe you could use an eazyplug pyramid mini, instead of soil next time, it would fit inside no problem, that is unless you are trimming the roots. You can see the pyramids mini in my grows.
Hey superquark!
Looking at the leaves, you might want to stop putting anything in the water..!
They are becoming burned where the plant is trying to get the excess nutrients as far away from its core as is possible!
Since you appear to be watering often, and feeding at least once a week - seeming to only use the water when you are trying to adjust the pH - perhaps try watering only a little every other day without added nutrients or supplements for a week or so.
The soil has probably retained a lot of extra nutrients, which is driving the pH down! It started a full point above where it is now!