The Red Sea Magnesium pro test kit is a little different.
According to the instructions, the end point is the FIRST time it turns blue. It will turn purple again after a while, but you shouldn't add anymore titrant. I didn't know in the beginning and was "testing" 1500 or 1600 ppm mg.
I swapped to Salifert for MG now, and the end point is instantaneous and unambiguous, so I'm pretty happy with that.
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my reef tank is the best skimmer... of my wallet
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