If you are having such problem and pwc
does not correct it, I would suggest to test first the parameter of your salt water mixed before pouring it into your tank. Each brand of salt mix has different amount of elements. I was having same kind of problem before with alkalinity when I was using the Reef Crystals by Instant Ocean. When I started using another brand it corrected the problem and increased my calcium level. I did took the readings using Salifert test kit and here is the result of IO.
I took another readings with API test kit and the dKH
is about 1.5 points higher (13.00). If you have similar readings, it explains why your dKH
stays on the upper borderline but once you have lots of water surface agitation your PH will go up and dKH
will go down.