Nitrogen is a critical element in aquatic environments, existing in various forms including ammonia (NH3), nitrate (NO3-), nitrite (NO2-), and ammonium ion (NH4+). Of these, ammonia is known to be the most toxic to aquatic biota such as fish and amphibians, followed by nitrite and nitrate. Nitrate, as the final oxidation product of the nitrogen cycle in natural waters, holds significance as the sole thermodynamically stable nitrogen compound in aerobic aquatic environments. This paper provides an overview of nitrogen forms in aquatic ecosystems, emphasizing their ecological implications and potential risks to aquatic life. Drawing on insights from Rouse et al. (1999), the paper elucidates the hierarchy of nitrogen toxicity to biota, with ammonia posing the greatest threat followed by nitrite and nitrate. Additionally, it discusses the role of nitrate as a stable nitrogen compound in aerobic waters, highlighting its importance in nitrogen cycling processes. Through a comprehensive examination of nitrogen dynamics in aquatic environments, this paper aims to enhance understanding of the ecological implications of nitrogen pollution and its impact on aquatic ecosystems. It underscores the importance of monitoring and managing nitrogen levels to mitigate adverse effects on aquatic biota and safeguard water quality.