This is also true for your plants. While plants do need nitrogen, preferably in the form of ammonium, as a food source, the amount of nitrate available in the average tank is many times greater than the amount that would be found in the water where the plants were collected. Elevated nitrate levels will cause severe algae problems long before the fish show any adverse effects. This is also true of phosphate. Although there is more than enough nitrogen and phosphate available for your plants in one form or another, there are other substances which may need to be supplemented.
Aquatic plants obtain nutrients in two different ways.
Almost all are able to absorb some nutrients through their leaves, directly from the water. Many also obtain nutrients from the substrate through their root system. For this reason is important that the proper nutrients be available both in the substrate and dissolved in the water. If you do some reading, you will find that there are almost as many methods of fertilizing plants as there are experts. There are even a number of articles available that have in depth analyses of the chemical composition of fish waste according to the diet of the fish.