It just drives me crazy. When I did a startup of my 10 gallon I took filter media, gravel, and water from my 75 gallon, let it run for a couple of days while "feeding it" and then tested it. Ammonia and nitrites appeared to be at zero so I added two platies. A day or two later I did another reading which looked like zero to me but my wife insisted the ammonia was .25. I did a 25% water change immediately and then 2 days later tested it and she said it was zero. To me, I never saw any change in the test colors at all. It was really frustrating because I have all this knowledge but feel like I'm being held hostage by these darn testing kits. The testing kit is fine, it's the ones from Aquarium Pharmaceuticals and I even brought my water to the lfs for them to confirm the readings were zero.
