Definition: Nitrogen is the chemical element (N2), a xolorless, odorless, and tasteless gas comprising most of the atmosphere and present in thousands of biologically important compounds. In aquariums it is present in various organic and inorganic compounds that arise during the bacterial breakdown and conversion of proteins.
Proteins enter the water by means of fish food and dying plants and animals, and other nitrogenous substances find their way into the water via fish excreta. By the interaction of various kinds of bacteria they all are broken down in many intermediate stages to ammonia/ammonium, then through nitrite to nitrate (in denitrification filters, even to gaseous elementary nitrogen).
Proteins enter the water by means of fish food and dying plants and animals, and other nitrogenous substances find their way into the water via fish excreta. By the interaction of various kinds of bacteria they all are broken down in many intermediate stages to ammonia/ammonium, then through nitrite to nitrate (in denitrification filters, even to gaseous elementary nitrogen).

