One suggestion is that if anyone ever doubts the accuracy or viability of any test (or a new box), use a standard control. I use I.O. @ 1.025 (mixed overnight) for alk, Ca, and Mg..the values are very close from batch to batch. There is also published values.
Ever since the I.O. Alk "incident" I test every new bucket of salt for Ca, alk and Mg to make sure something hasn't gone off and as a side bonus I control my test kits every new salt bucket....for me about every 4 months. Comparing values between test kits and/or any friends parameters should work the same way.
Personally,I like Salifert for their accuracy, even though they are a bit pricey. I have seen the Hagan Calcium test be out 20-40ppm (depending on the color shade you decide on) , maybe fine for spot tests, or in combo, but I'm to uptight for that
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Just my 2 cents, hope it's helpful
Cheers