Is he showing any other signs of stress such as breathing heavey, swimming irradically, red blotches or swolleness.
How often do you clean out your filters? You should only do one at a time as to not disrupt your nitrifying bacterial colony too much. I would get a new test kit as it should show one of the colors on the color chart unless it is outdated or the test is not being performed properly (which I'm sure is not the case).