The main toxic action of nitrate on aquatic animals is
due to the conversion of oxygen-carrying pigments (e.g.,
hemoglobin, hemocyanin) to forms that are incapable of
carrying oxygen (e.g., methemoglobin) (Grabda et al.,
1974; Conrad, 1990; Jensen, 1996; Scott and Crunkilton,
2000; Cheng and Chen, 2002). Nevertheless, owing to
the low branchial permeability to nitrate, the NO
3 uptake
in aquatic animals seems to be more limited than
the uptake of NHþ
4 and NO
2 , contributing to the relatively
low toxicity of nitrate (Russo, 1985; Meade and
Watts, 1995; Jensen, 1996; Stormer et al., 1996; Cheng
and Chen, 2002; Alonso and Camargo, 2003).
from
http://www.waterboards.ca.gov/water_iss ... ef2426.pdf

there's a newer study out on nitrate toxicity in fish, but i ain't shellin out 41 bucks