The best replacement for kalk is calcium reactor.

The best alternatives to kalk are rated as following:
★★★★★  calcium reactor
★★★★★  2 part
★  two part
★  2 part dosing
★  kridt

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Speaking about kalk, one probably shouldn't forget about ato.

Other search terms related to kalk and their ratings are:
★★★★★ ato
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kalk
calcium reactor
2 part