Unless you can find another source of nitrate that can refill the tank with nitrate as fast as you do a water change, then I would suspect that fertilizer stick.
If the only NO3 really is in the water, then as big a water changes as you do cuts the nitrate that much. 50% water change = 50% reduction in NO3.
If you do that much water change and do not see a reduction then there are several possibilities:
Bad test kit.
User error with the test kit.
Hidden source of NO3 that is released when you refill.