Nitrogen. Used in large quantities by aquatic plants, Nitrogen is one of the three main nutrients (called macro nutrients) central to plant growth. It is usable by aquatic plants in the form of Nitrate, Ammonium and Urea.
Nitrogen is one of the most basic and critical building blocks of plant protein, and is crucial for production of chlorophyll, nucleic acids and plant hormones. It is the macronutrient that is absorbed in the largest amounts by aquatic plants.
Uneaten food, animal waste and plant rests release ammonium, and filter bacteria convert it to nitrate. However, in tanks with low to medium bio-load and reasonable plant mass nitrogen typically needs to be supplemented with fertiliser. This is especially true in high-tech aquariums.
Nitrogen supply can be limited to reduce chlorophyll production, enhancing red colouration in some plants. However, this method is potentially dangerous as plant growth can also be limited, leading to plant stunting or death and subsequent algae outbreaks.