pH at 7.0 is hard to believe. Are you sure it's not higher? That should be a measurement error.
The alkalinity might be dropping due to the lanthanum chloride or increased growth. Some lanthanum will precipitate as lanthanum carbonate.
Kalk is a fine way to add calcium and alkalinity, but I'm not sure I'd have two experiments in progress at the same time. I'd probably just add a bit of baking soda to get the dKH up to at least 7 for the daily low. Maybe the tester is a bit off, but the number seems to be reasonable.
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Jonathan Bertoni
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