Good options for removing nitrates are fast-growing stem plants like Hygrophila or anacharis. Their growth, like all plants, is largely dependent on the amount of light available, however. If you're seriously looking into a denitrating filter, do a search for "coil denitrator". Its a type of filter that grows anaerobic bacteria to promote denitrification. However in most cases, plants and/or water changes are a far easier option.