Which one would you believe? All tested within 30 minutes of each other.
API - 390
Salifert - 420
Apex - 433
API - 390
Salifert - 420
Apex - 433
Calcium Testing Difference
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Randy Holmes-Farley
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Impossible to say, but I'd expect the value is likely in a range around 420 ppm, and is fine. 
Luckily, the OK range for calcium is huge.
Luckily, the OK range for calcium is huge.
