I assume many/most know this but I did not put the pieces of information I had together properly.
Since adding the scrubber, and the initial PH spike, PH continued to drop until today (still around 8.1). I have observed this and initially thought the media would get weaker and I was disappointed how quickly this happened, but the media still shows its ‘effective’ color, so I thought more about other reasons for the drop.
There is a need to add one teaspoon of water to the bottom of the scrubber to keep the humidity higher which the media needs to work properly. This evaportated by now so I replaced this and added a reminder to check this every four days for now. However, since I like 'wingdings', as KReefer stated above, I had already ordered a 3d printed cover for my skimmer from Premium Aquatics to lead the humid tank/skimmer air back to the scrubber which is supposed to make the media last longer. Has not arrived yet so need to manage this 'manually' for now.
Still, I did not think the lack of one teaspoon of water would make such a drastic difference - drastic meaning 0.1 PH decline or slightly more compared to a few days ago, all things equal (relatively).
So I gave this more thought and checked alkalinity one day earlier than my weekly check. Yes, I check this only weekly if I feel it is relatively stable and switch to every 3-4 days if I am making changes and see how they play out. I know you would say why did you not check alk immediately, and the answer is that my alk had been (slightly) higher than my target alk for many weeks now, and I had it dialed in so it would be pretty much around where I wanted it but slighly higher. Also, I have been dosing daily 3 ml of Sodium Silicate (for Dinos) so I thought that an alk decline would not be my issue (rather the opposite).
Turns out that Alk was at 7.4 (Hanna), down from 8.6, 6 days ago. The single largest drop of alk in a week for a long time or ever since I am measuring alk (would be good to have an APEX to measure this daily...haha). As per the last two ICP tests from Fauna Marin ICP Total, Hanna appears to overstate alk by exactly 0.6 for both ICP measurements 30 days apart. Could be my testing, but as previously mentioned, I try to do this very properly. Regardless, my measurement (or Hanna's) overstates alk by 0.6, so I would be at around 6.8 actual alk right now, which I assume would explain the PH drop.
So what has been driving the larger alk drop? I would say two things: I lowered the daily Kalkwasser addition by about 10% last week as I wanted to slowly and slightly reduce my higher calcium levels, but I was also hoping that the other explanation is that the higher PH drove up alk consumption (and growth?).
Adding this all in here for my own record but also for other insights and commentary, probably an easy explanation that higher PH leads to higher alk consumption but I did not know, specifically in the context of using a CO2 scrubber.
, but it gives me an opportunity to correct my previous post 
