Glen ur calcium reactor is not causing the PH to be at 8.4
i think u need to recalibrate ur PH probe.
if im understanding this correct ur PH was all ready at 7.9 & u are now worried about ur PH been 8.4, get this through ur head it's not the reactor, not only did u hook up a reactor but u also hooked up a reefkeeper2 so my guess is it's ur probe , try cleaning it, & dont loose any sleep over it, if ur PH starts to drop to 7.5 7.2 7.1 then yes u have to much CO2 melting media to fast & with that nice reefkeeper u got it will shut the co2 off before it gets that low, dont add anything to try & raise ur PH let ur tank water be for now, some times in the winter month ur PH drops because the water lacks oxygen this might be ur problem.
& i mean this in the nicest way.
UPS sucks