I would use Salifert not API
I have seen API off by as much as 4 points.
API is really really random.
I do not trust it for Alkalinity (The same goes for Calcium - I have seen it off by as much as 120 points).
I would use Salifert for Alkalinity, Calcium, and Phosphate.
API is fine for the basic tests though.
How do you know which one is off?