Nitrate is a plant food and is normally rare in situations where plant growth is abundant such as ponds and natural water bodies - this is why it can be so deadly to wild-caught fish or sensitive species. In heavily planted freshwater tanks or marine aquaria with lots of liverock or a deep sand bed, some degree of balance can be achieved as long as patience is exercised in the early days of stocking. More usually, nitrate accumulates as a result of healthy biological filtration and is best removed by regular partial water changes.
By performing a weekly 25% change you can limit the build-up of nitrate, although overstocked systems or those with large messy fish may need more frequent changes. It is always less stressful for both fish and fishkeeper to perform lots of small changes rather than a few large-scale ones and in the case of extremely high readings, daily water changes can be carried out until levels are under control. Remember that established fishes can tolerate levels that can be damaging to newly introduced livestock and when problems occur introducing new fish, this is often the cause.
It is well worth testing your tap water for nitrate, as this often contains concentrations that exceed acceptable levels and in this instance, it may prove impossible to run your aquarium using your mains water. In this case it may be necessary to investigate alternatives such as R.O. water or nitrate-removing resins, etc. Often a solution can be tailored to your aquarium after discussion with your regular store.