A friend of mine has a planted sump filter; live plants utilize nitrates, and will help keep nitrates in check.
Ammonium as well
In aquariums both fish and bacteria continuously release ammonium as they metabolize food and organic matter, most aquatic plants (and algae) vastly prefer ammonium over nitrates as their nitrogen source. This means that plants continuously sift the water for ammonium and its toxic component ammonia. Ive never had problems with ammonia in my planted aquariums.
One plant I like to use in my tanks to suck up nitrates is java fern. This tough little guy will grow in about any conditions. Low light, low nutrients, and rough fish. If you attach it to drift wood it will grow to it. It got tough leaves and silver dollars and pacus rarely nibble on it. It's a pretty good canidate in most tanks.
Cheers, Matt
Any healthy growing plant works to some extent but as Harley says, the plants like water hyacynth and water lettuce work best. If you want submerged plants that work well ant fast grower like elodea, java moss, and java fern are good. Just remember that they only hold the nitrates while living and will release them if the die. With fast growing plants trim off the excess growth and discard it, this exports the nitrogen products out of yor tank permenantly. The trimmings can be fed to animals like rabbits or tortoises, used as garden fertilizer, or if you grow enough of them, traded for credit at some lfs.
I think Java moss might help pull some of your NO3 out of the water. It seems to me that when I use it my algae problems are drastically reduced. I think that it simply out competes the algae. . . and it looks great doing it.
ive read the water lettuce is one of the best for removing high amounts of nitrate just cant get my hands on any none of the lfs carry floating plants where i am.