7.5 DKh isn't that bad, but it is in the low range of what you want to shoot for.
As for pH, how is it measured? Are you sure it's accurate?
Is there anything in the system that could be leaching and making pH run high? That's awfully high for a system running a Ca reactor.
Do you use frag plugs or anything? Some commercial brands can drive your pH up, since they are rarely (if ever) cured long enough IME. Five frag discs in my 120 gallon tank drove the pH up 0.2 for a few weeks...