Did you use any live rock? If so, are you blasting the rocks with a power head, stirring up or vacuuming your sand bed, cleaning your sump, rinsing any sort of mechanical filtration on a regular basis etc? Your nitrates are probably high because your tank is working overtime to process all the crap within it. Until these things are removed your just spinning your wheels with the water changes IMO.
Just as an example, depending on the size of the tank, if a fish was to die you might see a nitrate spike. Until that fish is either removed from the system or consumed by the CUC it will continually be a thorn in the tanks side.