@Eli -- My tank never reads above 5ppm Nitrate, which is incredibly low. Do these plants remove NO3 from the water? Absolutely. Lots of plants plus good filtration does wonders for water quality.
@Le Clown Blanc -- Sand is completely inert, it cannot "go bad". It can form anaerobic pockets but that doesn't happen often and it usually happens when you have several inches of substrate. It is not anything I would concern myself with.