REVIEW OF: http://www.atlantech.ca/public/articles ... uality.PDF

Well, I call into question the quality of the research because as I read it certain things (based on my knowledge of water chemistry and fish biology) just screamed out at me. However, please note the researcher calls these same issues into question; therefore, I am not calling the quality of the researcher into question just the experiment itself. This happens A LOT in science. The experiment doesn't go the way you expected because, in the end, it is an experiment. Sometimes your control and changing of 1 variable ends up changing multiple variables. Sometimes you just make stupid mistakes and introduce error into your experiment (as is the case here). This can yield results that are not useful or not relevant to the original research question. I don't mean to painfully go over the fundamentals with you, but I have to preface my response with that...

A lot of the symptoms scream out to me nitrite poisoning, but in the discussion the researcher addresses this:
When nitrate levels were elevated due to addition of a nitrate salt or from nitrification, increased nitrite levels were observed. This increase in nitrite was most likely due to enhanced reduction of nitrate to nitrite caused by the elevated nitrate concentrations. (page 5)
In the end, I don't think toxicity was shown because the nitrate levels used did not kill the fish. The exception is the first experiment performed, but as the researcher stated it could have been caused by anything including ammonia or nitrite. If you notice when he examined a RAS with nitrate over 200 mg/L (ppm) the fish were not all dead and in fact in a commercial system. Clearly, something went wrong in his first experiment if there are commercial operations that don't have dead fish at these levels. To shed light on this I would like to explain there are bacteria that convert nitrate back to nitrite. In systems that have properly established cultures, the direction of this conversion is vastly in favor of nitrite to nitrate, but in his experiment it may not have been. This is further supported by his findings where the symptoms resemble nitrite poisoning. Coincidence or not, it should definitely raise some questions in your mind as it did for the researcher.

The changes observed in the nitrate treated fish most likely represent a pathological response as apposed to a generalized stress response. (page 6)
Again, the researcher has concluded that although nitrate MIGHT cause problems in RAS, it was not due to a nitrate toxicity. Instead, it was a secondary infection that did the fish in. As I stated earlier, I have seen scientific research discussing possible decline of immune system response in high concentrations of nitrate.

The water quality in this report was not mentioned, so it is not known if the nitrate concentrations were elevated. (page 6, end)
I was very disappointed when I read this. Maybe chloramine existed in the water and released large amounts of ammonia, chlorine, and nitrite into the water. Maybe the water contained other toxins that caused danger to the fish. I think you and I both know that pouring water out of your tap and then throwing some fish in is just a BAD idea. TSK TSK

Conclusion
The data presented here support the theory that prolonged exposure to elevated levels of nitrate may decrease the immune response, induce hematological and biochemical changes indicative of a pathologic response, and may increase mortality. If elevated nitrate levels are responsible for the pathologic changes seen in these fish, then management of recirculating systems must change to lower nitrate levels. The pathologic changes are sufficient to affect the normal physiology of the fish and will probably result in decreased growth and increased susceptibility to disease. These results however do not conclusively show that elevated nitrate levels are responsible for the pathology seen. Further studies demonstrating a dose response to nitrate levels should be conducted prior to making major management changes in a recirculating system.
Ultimately, I think this study is pretty bad. The experiment was poorly designed, and the researcher knows this as he describes all the flaws and leaves some out. Moreover, the researcher has no confidence in his findings nor can he make recommendations based on his findings. This is, in my opinion, not evidence to support nitrate toxicity in fish nor does it support the theory that nitrate causes a reduced immune response in fish (but perhaps does raise interest in the latter matter).

I think you will be hard pressed to find any evidence that supports nitrate toxicity in fish, which is why it is often presented as non-toxic in many textbooks. You have found one such textbook, and I am currently reading another titled "Aquaculture An Introductory Text (2nd edition)" and it was recently updated in 2009; so, I believe it to be up-to-date.

Please continue searching if you have the time and interest. I think you will find the process educational and enlightening. You will not regret it, and you will be better off for it. If you'd like, I can PM you a list of scholarly databases that are FREE to the public. I found they are about 10 years behind, probably why they are free, but that's not bad for us in aquaponics because we don't go into anything in depth and science has answered a lot of issues we face, at least to respect to aquaculture, hydroponics, and botany. I use other sources as well, but I have access to them as a student and it's not available to the general public, unfortunately.