For a long time, I would get zero readings with the tube over the card, but my pH would be low if held over it. With the tube against the card, I'd get 8.2 for pH and 5-10 Nitrates. I wasn't sure which was which, but I knew one of my readings was less than perfect!
Finally got official word from Aquarium Pharmaceuticals on the correct way to read their kits (tube hel above or against the card):
"Every test, besides copper, requires you to read results by physically touching the test tube to the color card for comparison. With copper, you still touch the the tube to the color card, but you look down through the tube from the top, not from the side."
Since a week or two after removing my HOB
, my 'trates read zero even when against the card. Yea me! =D