Mike, I've been saying this for years.
Example, The Seachem calcium test kit and reference sample match to a tee. But they are both off when compared to other test kits.
Lamotte, Salifert and API all come in close, in my testing, yet Seachem is always around 70 ppm lower than those mentioned.
I do not know whether it is even possible to make a calcium standard. Randy would know.
But the proof is in the pudding. You buy 4 hobby grade test kits and 3 of the match. The fourth doesn't. And yet the fourth is the only one that comes with a reference.
You be the judge.
