Nitrate is being produced constantly as the end product of the nitrogen cycle. Detritus and other organic matter increase nitrate levels. Dirty filters, over-feeding, over-population, dying or decaying plants are also contributing factors.
Tap water may another source of nitrate introduction to the aquarium.
Nitrate levels should be below 10 ppm in fish only and planted tanks. For reefs it should be below 5 ppm, as accumulating nitrates do have an effect on delicate corals, anemones, inverts and crustaceans. Nitrates above these levels will be a feast for algae.