Those who run a Ca reactor and a Kalk reactor at the same time, normally do so to compliment each other. Where the Ca reactor maintains the availability of Ca and various other ions as the media is dissolved, the Kalk reactor tends to maintain hardness, and helps scavange/counteract excess CO2 released in the Ca reactor effluent or produced as a metabolic by-product of aquatic life.
"Actually, having the kalk and sand reactor outputs in the same area works out rather nicely since the sand reactor effluent contains excess CO2, which is exactly what the kalk effluent needs to rebalance (the excess OH- in the kalk effluent will usually convert atmospheric CO2 into CO3-- and HCO3- depending on availability, the sand reactor output makes CO2 very available). The tendency of a kalk reactor to raise pH and depressing CO2 levels is completely offset by the tendency of a sand reactor to lower pH and raise CO2 levels. "
Depending on the size of a fluidized bed filter (I assume this is what's meant by a "sand filter") it is doubtful that a substantial anerobic effect will develop across the bed considering the flow-rate thru sand filters. OH- ions, in the presence of CO2, are MUCH more likely to form calcium carbonate, and precipitate out of solution as a solid. Based on this, a kalk reactor's effluent should be placed close to return-pump suction, but far enough away to allow mixing before re-entering the display's water column. There, it is more effective at offsetting CO2 in the display, and controlling/stabilizing pH in the tank.
The lion's share of "excess CO2" in a system is produced/introduced from plant/aquatic life off-gasing after the lights go out. Caulerpa, Xenia, and others stop producing oxygen as a photosynthetic by-product in the absence of light ...the CO2 they produce in darkenss is released into the water column. These sources and an improperly tuned Ca reactor are the MAJOR CO2 sources in a system. For this reason, Ca reactor effluent should always be directed through a refugium, if present in the system, or into a skimmer's intake to aerate it aggressively before returning to the tank. This way, less OH- ions are "lost" to precipitation and are available to buffer system pH.
Overall, considering a finite water volume, and even the hulk systems out there are finite. The thought that somehow a "localized build-up" of any element can occur is not realistic. To test this, put one drop of red food coloring into a corner of your sump, and count the seconds before it is completely imperceptible. ...Most reef systems have a turnover rate of 10-20 times total water volume per hour. It's pretty safe to assume a uniform distribution of ions with that kind of movement. So, anywhere for the effluents is okay.
It offsets the pH swing at night too.
