dmjordan is right, a water change will bring down nitrates. In fact, as nitrates usually will slowly increase over time in an established tank, water changes are the accepted means of lowering them and keeping them in check.
For a fish-only system, I understand 40ppm is about as high as you want to go, although of course lower is always better. If corals/inverts are a factor, then the upper limit I understand is around 20ppm, again lower = better.
Of course, a regular schedule of waterchanges is simply a very good thing to do as not only will they lower nitrates, but also replenish many important trace elements (presuming your salt mix contains them, as many do although good idea to make sure).