The best way to get rid of nitrate is through frequent water changes (easily done with a gravel vac). In freshwater aquariums live plants will help. In saltwater aquariums protein skimmers and refugiums will help. However, nitrate often persists and nitrate levels as low as 5 ppm can cause algae blooms. Nitrate can also cause stress to fish and invertebrates, and lead to diseases and reproductive failures.
If nitrate persists, the aquarium nitrate removers on this page will help to keep your nitrate levels manageable.