I'm a little tank---54g, and the kalkwasser from the ro/di tub is quite enough for me. Holds my alk at 9.3 and my cal at 480. I run lps, sps, and a clam, of my high calcium users. I'm coming back after a house move, in which I lost most of my sps, started kalk this summer, fought a balky KALK reactor for months, settled on this simpler method, and now have rapid growth starting: heads of lps dividing like crazy and montipora starting to grow a number of growth-heads. No crankier sps, yet: they all perished. If I can get the montiporas to grow, I'll put in some ac. valida or bali slimer.
If there were more corals, a tank of the month candidate, I would have to use a calcium reactor. If I were a BIG tank like yours and packed to the gills I might have to use both a calcium reactor and a kalk reactor.
Again, as I understand it, and I'm no expert: a kalk reactor LETS kalk [lime/calcium] powder dissolve in ro/di, which it does readily, and shoots it out into the tank.
A calcium reactor uses the injection of CO2 from a cylinder of gas in order to FORCE the dissolution of calcium beads into the water, so there's no practical limit, I suppose, to the amount of calcium you can feed in---except that it will affect ph. So does kalk, but I believe they affect ph in opposite directions: somebody check me out on that.
The fact that there's no practical limit with a calcium reactor means, too, as I dimly understand, that OD'ing with a calcium reactor is more dangerous than OD'ing with a kalk reactor.
A kalk reactor, too, works by topoff: how much goes in is dependent on your evaporation. At 1 gallon a day, my 54 g sucks in a nice amount for stability. But the amount of kalk that WILL dissolve in ro/di is a set and stable amount, so no overdose, even if the evap rate increases.
With a calcium reactor, because the calcium is FORCED into solution, this is not evaporation dependent, again as I understand.
My little tank at full stretch was sopping up 2 teaspoons of calcium a day: this made my leaving it pretty difficult, when I was hand dosing. And at 18.00 a jar, it was expensive. Not to mention the buffer.
Kalk, at 5.00 for 2 pounds, is cheaper. I dump in a pound [steady rate of dissolving in ro/di, no great need to measure, really] and that will do it for some number of days---up to a month, as my tank is now: probably a couple of weeks, once I get it going the way it was.