There is some real confusion in this thread concerning plants and nitrogen.
Most aquatic plants, and overwhelmingly the majority of those we use in our aquariums, take up ammonia/ammonium as their preferred source of nitrogen. They do not take up nitrates unless the ammonia/ammonium is depleted and then provided the other nutrients are still adequate and the light is sufficient to drive photosynthesis. We are here referring to natural or low-tech method planted tanks, which is what most of us have. Things are different in high-tech method planted tanks with brighter light and diffused CO2 and nutrients have to be in balance; ammonia/ammonium from the fish and decomposition may well become exhausted in such tanks, and adding nitrates is safer than adding ammonia.
Scientific studies have shown repeatedly that the vast majority of aquatic plants greatly prefer ammonium over nitrate. Moreover, they prefer taking it up via leaf uptake from the water, rather than root uptake from the substrate. The ammonium preference of aquatic plants is substantial. For example, when Elodea nuttallii was placed in a mixture of equal parts ammonium and nitrates, the plant removed 75% of the ammonium within 16 hours while leaving the nitrates virtually untouched. Only when the ammonium was gone, did the plant begin to take up nitrates. Likewise, when the giant duckweed Spirodela oligorrhiza was grown in nutrient media containing a mixture of ammonium and nitrate, it took up ammonium rapidly, whereas it virtually ignored the nitrates. Because the plants for this particular study were grown under sterile conditions, the ammonium removal could not have been due to nitrification. Also, the investigator showed that plants grew rapidly during the study suggesting that N uptake was due to the plant’s actual use of this major nutrient. Nitrate uptake does not occur until plants are forced to use it, that is, when all ammonium is gone. Even then, there is a delay, because the setup for nitrate uptake must be generated first. Thus, Water Lettuce required 24 hours to attain its maximum rate of nitrate uptake. [cited from Walstad who provides links to the various studies.]
All plants use the N from ammonium—not nitrates—to produce their amino acids and proteins. If a plant takes up nitrate, it must convert the nitrate to ammonium in an energy-requiring process called ‘nitrate reduction.’ Plants must expend essentially the same amount of energy (83 Kcal/mol) that the nitrifying bacteria gained in order to convert nitrates back to ammonium. The overall reaction for the two-step process of nitrate reduction in plants is: NO3 - + H2O + 2 H+ ⇒ NH4 + + 2 O2. The energy required for plants to reduce nitrates to ammonium is substantial, equivalent to 23% of the energy obtained from glucose metabolism. [cited from Walstad; and it is aquatic not terrestrial "plants," just to be clear before someone tries to muddy the waters.]
The above clearly shows that nitrates at zero is not going to affect plant growth--again, in natural or low-tech method tanks. But it will benefit the fish. Many of us with low-tech planted tanks have very low including zero levels of nitrates (using our hobby tests, which admittedly may not be scientifically accurate but reading zero nitrates is still very low nitrates}. No fish we keep in our tropical aquaria come from natural habitats that have nitrates above a few mg/l (ppm), and many are zero. The nitrates in my tanks, which are fairly well stocked with fish, have been in the 0 to 5 ppm range using the API test for over twelve years, never rising. My Echinodorus plants thrive in my tanks.
