Trouble reading Nitrate tester

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Aquarium Advice Freak
Aug 1, 2006
Dirty Jersey
I have the API nitrate tester. My problem is, if I hold the test tube vertically up against the card, it looks yellow (0ppm nitrate). If I hold it horizontally, it looks a little orange (5ppm). And if I take off the cap and look straight down, it's even more (10ppm).

What's the right way to read it?
As long as it is like the API kits I know and love, just hold it over the blank white area of the card in a well lit room.
That's exactly what I'm doing. But what's the proper orientation to hold the tube against the white card? I get three different readings depending on which way the tube is oriented against the white background.
I've always held it with the length of the card. The only other test I've ever seen done differently is copper.
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