Testing question

[swaziland]

I have a dumb question, I just purchased a nitrate salifert testing kit,it is always coming outat the purple color even after a water change.(100) does this mean this is ok? My other test kit from a differant co. is telling my I have no nitrates, HELP, Not sure how to read the Salifert card?

 

[trainfever]

Try testing a batch of clean water before doing a water change. If the new water tests high for nitrates then you know the test is defective.