I tend to have a similar problem..
but lucky for us, nitrates arn't nearly as bad as nitrites. so I just eventually gave up and deal with it. Try not to overfeed, do weekly water changes (when doing so, only vacumn 1 half of the tank 1 week, other half the next, so I don't take all the good stuff too), and perform filter maintance as needed...
I do use from time to time de-nitrate from seachem with some varied results.. Also I beleive tetra just came out with something called nitraban, picked some up but haven't tried it yet. Also keep in mind with most chemicals, you won't get accurate test results. because of the way the chemical reacts and works. You will proabbbly still get a reading on some things, though its not theree, or in small amount, but not in a harmful state. after chemicals, if you want a accurate reading, wait a few days...