I seriously have to disagree with the levels required to cause adverse affects in fish.
A group of Spanish chemists have published a review of the toxicity of nitrate upon aquatic organisms which includes new data on the effect of nitrate upon freshwater invertebrates.
Camargo, Alonso and Salamanca of Madrid's Departamento Interuniversitario de Ecologia at the Universidad de Alcala, published their findings in the latest issue of the journal Chemosphere.
The review, which looked at marine and freshwater invertebrates, fishes and amphibians, suggests that the main toxic effect of nitrate is due to its ability to convert oxygen-carrying pigments to forms that are incapable of carrying oxygen.
A similar process occurs with nitrite, in which the chemical oxidises haemoglobin to methaemoglobin which has no oxygen-carrying capacity.
The authors claim that nitrate is more toxic at higher levels and when animals are exposed to raised levels for long periods of time. Interestingly, nitrate appears to become less toxic the larger an animal becomes and is also less of a problem to marine organisms and to those adapted for nitrate-rich environments.
The paper also explains that even fairly low levels can have adverse effects if organisms are exposed to them for long periods.
Just 10ppm of nitrate-nitrogen (that's a mere 3.03 ppm of nitrate) has been shown to have adverse effects on salmonids such as Oncorhynchus mykiss, Oncorhynchus tshawytscha and Salmo clarki, as well as upon a number of freshwater invertebrates and frogs.
Marine organisms, however, are much more tolerant and levels of 20ppm nitrate-nitrogen (that's still just 6ppm nitrate) were required to cause adverse effects.
For more details see the paper: Camargo JA, Alonso A, Salamanca A. (2005) - Nitrate toxicity to aquatic animals: a review with new data for freshwater invertebrates. Chemosphere. 2005 Mar; 58(9): 1255-67.
