Ron Shimek proposes chronic exposure to nitrate levels of 100ppm+ is acceptable for both fish and inverts.
You can find more on
Ron's opinon on nitrates here.
(you can also find Ron's attitude towards dissenting views.)
My understanding is nitrate, in it of itself, is not a very toxic compound; Ammonia and nitrite are. However, its presence is generally indicative of other (possibly more virulent) problems. In high quantities, nitrate has been attributed to observable decline (e.g. growth rate, mortality, etc.) in invertabrate health. In fish, it is plausible to speculate high nitrate levels may also adversely affect their health. Fish employ bacteria and/or specific enzymes in their stomach for digestion purposes. A side effect of digestion may be the reduction of nitrates to nitrites. Nitrite, absorbed in bloodstream, will oxidize red blood cells and impede its ability to transfer oxygen. This leads to poor health or death by asphyxiation. This is speculative (I am not aware of any studies done on this), but I will present this as a possibility nonetheless. From personal observation, fish do not feed as voraciously in high nitrate (>50ppm) environments. Fish also exhibit more rapid respiration in presence of high nitrates (the aforementioned asphexiation, perhaps?). Decreased reproduction rates and egg mortality are documented consequences of high nitrate levels.
Personally, I don't think statements like "Levels upto 100+ ppm are fine. Nothing to get one's shorts in a twist about either way" (per R.Shimek) should go without specific qualification. It's an incomplete and irresponsible suggestion.
Anything under 20ppm I'd consider universally safe for inverts and vertabrates alike. I derived 20ppm from personal experience, anecdotal as it may be. Higher levels may still be beneign, but <20ppm is a safe target value.
[ January 21, 2002: Message edited by: Leonard ]</p>