I used to only test for Nitrite and Ammonia, never had any trouble. I got a test kit for Nitrates, and even after a 30%+ water change, it would read 40-60 PPM. No matter what I did, it never dropped below 40 PPM. The Nitrite and Ammonia always said 0 or close to it. I finally got fed up and got a new test kit. I now get readings of less than 10 PPM of Nitrates, so I must assume the first kit was reading wrong.
Chris