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low nitrate & phosphate values in rivers and lakes

Aquatic plants in nature derive the vast majority of their nitrogen and
phosphorus nutrients from the interstitial water of the substrate, not
from the water flowing above the surface. In nature you have very large
masses of soil and subsoil from which the nutrients are derived. In an
aquarium, you do not have the space for a high ratio of soil and
substrate to plant mass. Using overly rich soils in an aquarium is also
not a good plan because those nutrients are readily released into the
water since the CEC is not sufficient to hold more than a fraction of
it. I suggest that periodically fertilizing your substrate (for example
clay balls with fertilizer) is a very good way to ensure adequate P and
N for your aquatic plants.

I don't believe there is particularly much value in having a nitrate
level above 2-3 ppm except that if its 10ppm you can leave it alone for
a while without worrying about the level dropping too low. I dose
replacement water to about 5ppm nitrate and add fertilizer to the
substrate to ensure I don't run short of N but sometimes it still