The accuracy of API is doubted by some. For me, it was more the level of precision available (ie, they couldn't read low enough). For instance, the API phosphate test goes from 0 - 0.25 ppm
. However, anything less than 0.25 is impossible to determine, and you don't know the true value. With the Hanna Checker, I know my PO4
is around 0.01 - 0.02ppm. Same with the nitrate tests...API goes from 0-5ppm in it's first step. With the Salifert kit, I can test down to 0.1ppm.
Folks have done tests on API vs other brands with calibrated test solutions, and API seems to be middle of the road in terms of accuracy.