I'm sure the CaRX is one of the causes for the low pH. I also had a 'low' pH in my tank of Day:8.05 and Night:7.85. Much to my wife's chagrin, I ran a vinyl tube outside to my skimmer inlet and my pH is now at Day:8.20 and night:8.10.
I do not top off with Kalkwasser. But I may do that to get to a pH of around 8.3.
I know many folks will say not to stress about getting to pH to 8.3. And I haven't. My levels have been at 8.2-8.1 for a few months now. Reefing is a great hobby for those who like to procrastinate.