Kalk reactor no, because you feed it with RODI water with a metered pump and it needs a little pressure to work correctly. I use a Litermeter 3 for mine.
Calcium yes, most of them if not all. If it can be feed by gravity, then it can be feed with a pump. Actually it works better with a pump. Much easier to control.
Some use a tee and valve off their return. I use this and it works great!
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