I only use the Hanna checker for Phosphate testing, to get the desired accuracy (about every 2 or 3 weeks). I use API for Ca, KH and nitrates, not as accurate, but good enough for what I require. And I use Salifert for Mg.
Hanna checkers are expensive along with the required regents. Plus it is not as quick as a color test (like the API ones). I test my Ca and KH every few days (in case I need to adjust my dosers). I only test my Mg just before a water change, so I know how much Mg I need to mix up. I only test nitrates about once a month (always zero, with my bio pellet reactor running).
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