[Denis]

What's your Mg level ?
I'm surprised that your Alk drops so much with DSB.
Ca drop is somewhat large, hard to say.
The third part should actually be mag sulfate and mag chloride (2:3).
Alk swing like that are bad, you should dose more frequently (daily).
Alk 6.4 not a panic, but immediate action necessary.
My guess is that you Mg is low (~1000).
Aim for more stability, possibly invest in a dosing pump.
Hope this helps.