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Is there a difference?
API is a total copper test kit which reads up to 4.0 ppm. For the product you bought, you will want to maintain 1.5 - 2.0 ppm Cu at all times.
Salifert (and Seachem) copper test kit tops out at 1.0 ppm, as it is meant for low range ionic copper (Cupramine or pure copper sulfate). So, you'd never be able to accurately measure 1.5 - 2.0 ppm with either of those.