Ever since I started adding Kalk to the tank, I’ve been getting quite large pH swings.
To help combat the swings, I decided to put my refugium light on a reverse-daylight cycle. Currently it was running 24×7.
Looking at my pH graph for the last 6 months, you can see the point that I started adding the Kalk (the first jump up in pH for the top-off water). Prior to this point, both pH probes were in the tank and usually shadowed each other +/- .1 pH.
Around 5/1 I put too much Kalk in the top-off and it raised the pH in the main tank.
Where you’ll see the difference in the tank swing is in the last week or two since setting up the reverse daylight. The pH only swings a few points. The large dip is right after a water change. I’m not sure I let the salt-water mix long enough to have it properly oxygenated.