pH problems

I am currently experiencing issues with pH. Current levels are 7.9 at night and 8.05 during the day. Although not detrimental to the system, they are too low for comfort. I suspect the problem is due to closing up the house for winter.

Unfortunately, I can't run the Kalk reactor harder to counter this as it is topping off more than the evaporation and lowering my salinity. What to do...