I find it interesting that on the master test kit color card, where all the colors are columnized on a single card, the 10ppm nitrate and 20 ppm nitrate look identical. Yet, on the small (freshwater) card that comes with the single nitrate test, where nitrate alone is tested, there's a distinct difference in shade between 10ppm and 20ppm. The 10 is more orange, and the 20 is redder.
I also have trouble with the ammonia test. Sometimes the shade is more yellow and less green, but not true yellow nor the green shade for 0.25. It's in between. I have learned to allow for in between measurements. Occasionally, I do get a dull, true yellow, and it's no doubt that's zero.
Do you shake up your reagents before using them? API has admitted that the reagents can glop up at the bottom of the bottle, and if you don't shake them thoroughly so they mix thoroughly, you can get a false reading.
The pamphlet says to shake #2 nitrate reagent for 30 seconds before counting out the drops, and then when combined in the test-tube with #1 reagent, to shake the tube for a full minute, then wait five minutes for the color to develop.
But, the directions say nothing about shaking the other reagents. In truth, you need to do it with all of them, especially ammonia #1, because that is thicker, anyway.
Try shaking the reagents thoroughly before using them, and see if the color shades aren't a little clearer.
-- sirensong
I agree and I also shake all my bottles before using them, and then with the nitrates do as per directions. I find the 10 to 20 nitrates the hardest to decifer as well...