Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
If you have dry ferts, or any other fert that allows you to mix a solution known to be 10ppm, make the solution and measure it. Try 5ppm too.
Then it will become instantly apparent the proper way to read the results for any test, taking everything into account (vial position, lighting and angle, and your own eyes).
For the API tests, I found it to be with the vial pressed up against the white area of the test card, and with the bathroom lights (where I test) directly behind me.