[Delphinus]

I have one I suppose I could demo to you (I just don't happen to be using it, but if you need to see how it works I could do something) when I'm back home (out of town this week).

The main thing is that all they really all is a mixer, to help keep your kalk mixture saturated. Whether kalk can keep up your levels however is dependend on your calcium draw because you cannot add more kalk to your tank than the freshwater you lose by evaporation. So usually the best you can expect with kalk is a maintaining of current levels, or at least, a reduction in the decay rate.

So what a kalk reactor allows you to do is mix up a supersaturated solution (ie., part of the powder is unable to dissolve because it has nowhere to dissolve into), and thus the kalk that you add via the reactor is as concentrated as it can possibly be. As you add more RO/DI into the reactor, part of the undissolved kalk (the supernatant or whatever it's called, it's been a while since I did chemistry classes) is then able to dissolve, so the concentration of your effluent does not waver over time (until all the kalk powder is used up).

Calcium reactors are an improvement in that instead of feeding off FW for topup, they take water from your tank. Thus, you can run them as hard as you want, independent of your topup. But, they use CO2, this is complicated and expensive to startup, and CO2 pulls down your tank pH which can give nuisance algaes such as valonia a bit of a leg up (not always, but it can happen). So each device has it's own pros and cons. A kalk reactor tends to raise, or at least buffer the pH drop (ie., make the tank resistant to pH drops), so a few years ago it was pretty common to see serious SPS tanks running both Ca reactors and kalk reactors since they nicely complement each other.

But both tend to be a little old school these days. The current en vogue thinking is to use automated dosers or take it a step further and get into Balling method.