Just to make a couple of slight corrections to statements in two earlier posts...live plants can actually keep nitrates at zero. There is also bacteria in the substrate that do use nitrates.
On the plants: they assimilate ammonium as their preferred form of nitrogen [ammonia, ammonium, nitrite and nitrate are each differing forms of nitrogen]. They use a lot of it, and out-compete the bacteria in most cases. That means there is less ammonia for bacteria, so less nitrite and less nitrate resulting. Some plants will also assimilate nitrite and nitrate, but usually only when ammonium (ammonia) is used up. But depending upon plant load, type of plants, and fish load, all this in a healthy balanced planted tank can mean very low nitrates. This does not mean no water changes, which are still essential for other reasons. Without plants, nitrate is most easily removed by regular partial water changes, and half the tank volume changed weekly is a good level. I even do this much in my heavily-planted tanks.
On the bacteria, there are de-nitrification bacteria primarily in the substrate; these bacteria convert nitrate into nitrogen gas which is released back into the atmosphere. You can read the whole story in this article: http://www.tropicalfishkeeping.com/f...quarium-74891/
The above processes illustrate why a truly balanced aquarium can almost run itself. Making use of nature to do all the work that some aquarists expect from filtration equipment has its benefits.