Yes, I agree that they are not exact for any elements. However there is a wide degree of difference in accuracy in say, calcium or magnesium, compared to the accuracy of the test for iodine.
And, the problem is exacerbated by the fact that a little too much iodine and you can poison your tank, whereas a little too much calcium or magnesium might cause some precipitation but it's not a potential doomsday like the iodine MIGHT be.
I guess it boils down to first of all, if one believes dosing iodine to be necessary, and then if so, does one trust the test kit for that purpose.
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