Like the isoelectronic carbonate ion, the nitrate ion can be represented by resonance structures:
Almost all inorganic nitrate salts are soluble in water at standard temperature and pressure.In freshwater or estaurine systems close to land, nitrate can reach high levels that can poentially cause the death of fish. While nitrate is much less toxic than ammonia or nitrite, levels over 30 ppm of nitrate can inhibit growth, impair the immune system and cause stress in some aquatic species. However, in light of inherent problems with past protocols on acute nitrate toxicity experiments, the extent of nitrate toxicity has been the subject of recent debate.
In most cases of excess nitrate concentrations in outdoor aquatic systems, the primary source is surface runoff from agricultural areas that have received excess nitrate fertilizer. These levels of nitrate can also lead to algae blooms, and when nutrients become limiting (such as potassium, phosphate or nitrate) then eutrophication can occur. As well as leading to water anoxia and dead zones, these blooms may cause other changes to ecosystem function, favouring some groups of organisms over others. As a consequence, as nitrate forms a component of total dissolved solids, they are widely used as an indicator of water quality.
Nitrate also is a by-product of septic systems. To be specific, it is a naturally occurring chemical that is left after the breakdown or decomposition of animal or human waste. Water quality may also be affected through ground water resources that have a high number of septic systems in a watershed. Septics leach down into ground water resources or aquifers and supply nearby bodies of water. Lakes that rely on ground water are often affected by nitrification through this process.
So, it is reasonable for us as fishkeepers to keep levels of nitrate down. Although opinion varies as to which level it becomes harmful at, it would seem generally agreed that a level above 30-40ppm has a negative effect on aquatic life.
Source: wikipedia and internet
