Check the date on your test kit, and also make sure that you are viewing the tube against a white background. My tank in the kitchen (doesn't everyone have a kitchen tank) seemed to always have a hint of an ammonia reading, I finally realized it was the sage colored walls that were causing the test to look greenish. If my bottles have been sitting for awhile, I usually give them a good shake to make sure the chemicals are mixed before testing. If you can eliminate the tap as a source and testing errors, it pretty much leaves filtration as the culprit. I add ceramic rings to the grey plastic baskets in my emp 400 filters.