I don't know if this should go on the 'carbon' thread or not, but basically there are good reasons not to rely on the nitrate test kit as your sole measure of water quality. Certainly if I go without water changes and rely on skimming and biofiltration accumulation largely of phenols will cause water yellowing. To shift some accumulated wastes water changes and carbon use are required.... Starting with Eric Bornemans nice articles, and take your reading from there...
http://reefkeeping.com/issues/2003-12/eb/index.htm http://reefkeeping.com/issues/2003-13/eb/index.htm