Any ideas about low pH?

[nlreefguy]

For some reason I don't seem to be able to keep my pH up. It's always around 7.6, even with alkalinity consistently between 10 to 12. Calcium usually 400 to 450. I have supplemented alkalinity with kalk on occasion and brought up the pH temporarily but it always comes back down to around 7.6 even by the next morning.

It's an SPS dominated 50 gallon reef with a 25 gallon sump. Bubble Magus NAC6A skimmer, Chaeto in the refugium portion of the sump. 2 Tunze nanostreams 6045 for circulation in addition to an eheim 1262 for return from the sump. Fish include 2 percs, 2 lyretail anthias, a yellow tang and a royal gramma.

Nitrates are undetectable, phosphates same. Alk like I said usually 10 to 12, calcium 400 to 450. pH is the problem usually hovers around 7.6 and doesn't change much. I have tested with color kits and also have an aquatronica controller. Just changed and calibrated the pH probe.

Everything is healthy, don't get me wrong, but I have noticed that the corals are happier and grow faster when the pH is higher. I get much better polyp extension and growth then. I am considering just switching to kalk for ca and alk and to bring up the pH but I would also like to hear ideas about what could be keeping the pH suppressed. I was under the impression that if I was aggressive at keeping the alk up then the pH would stay high as well, but that doesn't appear to have been the case.

Anyone have any thoughts???