Also, what % water changes are you doing? Nitrates will be reduced in direct proportion to the percentage of water changed.
Example - nitrate levels at 100ppm
10% water change done, 10% of nitrates removed, nitrate level now at 90ppm
Assuming no new nitrates are added before the next water change, another 10% water change is done, reducing nitrates by 10%, or 9ppm. 'Trates are now at 81ppm
is done, again 10%, 10% of nitrates (8.1ppm) are removed, leaving them now at 72.9ppm
Final water change is done, 10% once again. Nitrates are lowered by 10%, or 7.29, leaving the remaining nitrate level at 65.6ppm
Much depends on the amount of the water changes, and if the nitrate level is rising again between changes.
Like astride13 said, we'll need complete setup details (along all tank inhabitants).