I have only ever used API, and I am getting ready to upgrade all my reef tests to salifert. IMO ATI is very hard to determine NO3 at low levels because the colors look so similar. I may be wrong about this but I think Salifert determines its level by the number of drops you use to change the liquids color. In other words, you get an exact number as opposed to guessing between 0 - 10 or so.