use good Ro/Di water and watch your feedings. Most of the nitrate removers advertised don't seem to work too well. I've had the same problem for a couple of years as I was using tap water.
A macro-algae(like caulerpa) in a refugium or algae scrubber will greatly reduce your NO3 & PO4, usually to 0.
It usually works best with the others.(LR, LS, DSB and/or PLenium)
Because of it's growth rate and hartyness caulerpa is the best, but because of those same reasons should only go in a refugium or algae scrubber, never your main tank(could become a weed). The better varities of caulerpa are grape, cup, mini grape, razor and blade.HTH
