You need to read the fine print:
Quote:
From Kordon/Amquel+ Website
This Standard Dose will remove (detoxify) at least 1.2 mg/L (= approx.1,2 ppm) of all ammonia compounds, at least 2.0 mg/L (=2.0 ppm) of nitrites, and at least 13 ppm (=13 mg/L) of nitrates.
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When they say remove they say right after that (detoxify) 13ppm of nitrates. What that means is that up to 13ppm of your nitrates in your tank per dosage will no longer be harmful to your fish, however, your test will still show the same amount of nitrate in your tank (as you've noticed

).