Randy Holmes-Farley
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My Tank Thread
Lou Ekus advises to dose A4R for Calcium not ALK maybe because of individual differences in atmospheric CO2.
The ONLY reason to dose AFR by calcium measurement is due to the lack of immediate testability of alk. It has nothing to do with CO2 being variable, and IMO is is a poor idea because calcium testing is too variable and the real declines in Ca to look for by testing are too small to reliably measure.
One can control alk, and to some possible extent, pH, using AFR or sodium bicarbonate when you want alk but no pH boost, and sodium hydroxide (or carbonate) if you want alk plus a pH boost.