Great advice above.
That 2 degrees of pH swing is actually fairly common from night to day. Real problems would only come if it went below 7.8 or above 8.5
No need to worry and feel free to adjust the photoperiod back to where you had it.
My first thought after reading your excellent description and ample data is that you meant 2-2.5 meq/L not ppm. Am I assuming correctly? If that is correct then the next thought that comes to me is that the test kit may be off. Have you checked it against another test kit or had Alk and Ca checked by an LFS

Next, if the readings are correct, have you added just the Alkalinity part and stopped the Calcium to see how the Alk might rise

FYI, the two part solutions do not have to be added together nor in the same exact amounts. In your situation, after checking the readings and finding them to be correct, when Alk is low, and Ca is within range just stop the Ca and continue adding Alk. until it gets closer to 10. (Calcium between 350-450ppm is within range, Alk range is 2.5-3.5 meq/L (8-14 dKH)) As you increase Alk, the Ca will start to fall. Continue adding the Alk and resume adding a little Ca solution to keep it from falling below 350ppm.
BTW, you are changing water more often than needed. No more than 10%/month works just fine for the setup you described. That will save you some time and trouble, making it easier to enjoy your reef aquarium.

Welcome to the WMAS. Just think, if you had asked about this perceived pH problem any sooner, you would not have had to go crazy.

Do you need any counseling to get over this craziness?
