A CO2-based Ca reactor will not drive down pH if it is properly adjusted and monitored , especialy if there is a secondary column that the reactor effluent must go through before hitting the tank circulation. Given a large enough reactor column and proper adjustment, it can supply any amount of calcium and alkalinity needed for a closed system, even in the presence of large dense populations of hermatypic organisms. The problem with kalkwasser reactors (Neilssen reactors) is that their ability to deliver calcium and alkalinity is limited by the rate at which water will evaporate from the system. Adding Kalk at rates faster than the maximum rate of evaporation will cause salinity variations as the kalkwasser dilutes the salinity down when addition exceeds evaporative losses.
Low pH with Ca reactors is often a result or excessive use of CO2 to dissolve the aragonite reactor media by circulating the recirc fluid at pH
< 6.7
+0.05 pH units, and this ultimately results in loss of alkalinity that would otherwise be used to build stony
coral skeleton (and the excess CO2 will bubble out of the system,
Le Chatlier's principle). If the CO2 delivery rate is properly adjusted, pH will remain in the tank at somewhere between 8.15 to 8.3 or so, depending on the numbers and population density for photoautotrophs in the tank and the number and density of heterotrophic creatures in the system. Every source of CO2, including the reactor, sandbed bacteria, nitrifying organisms(bacteria), fishes, high-CO2 room air, and nightime photosynthesizers (remember, they catabolize their CHO's at night = CO2 + H2O) all will have inputs on not only how much CO2 is produced, but on the ultimate level of buffer as any acids they produce will be potentials for alkalinity losses to maintain the tank's pH.
See some of the threads on adjusting CO2-based Ca reactors in the GRD (use Joe's name as the search name, I think there is a good thread on the setup and adjustments made to his current reactor's setup).
HTH
