All of the above. I've also found that making sure I don't have low mag has helped too.
I'd do it is this order:
Mag levels in line
Outside air to skimmer if able
Kalk
CO2 Scrubber (look up FishofHex)
Edit: Also, making sure that my phosphate is in line has helped a lot. Probably very indirectly, but thought I'd throw it in there.
Edit 2: Doing this has helped me get my pH to a pretty rock solid 8.3 - 8.5 range. Prior to that I was scraping along at 7.6.
This is the way, but I will add to get your salinity to 1.026 (35 ppt) which will help/fix mag levels. Then go to these steps.
