Fishkeeprian, if you're stable, nobody's using mg or cal at a rate that exceeds your water changes. You do not need kalk, but continue to test, because fish do use up minerals over time, and because if you have a clam or stony coral and that creature begins to build skeleton, (it can sleep quite a while, then wake up hungry) you'd be surprised how very fast it can strip-mine the water of calcium, throwing other parameters off. The basic sense is: stony corals and clams use more calcium than your water changes can supply, and once it goes deficient, a lot of things happen to the parameters. And b) do tests weekly: slow changes left unexamined can go critical-short eventually and cause you headaches. The simplest, easiest to run, and most predictive of all mineral tests you run is alkalinity: if it's right and stable, so are other things.