High nitrate levels create dangerous water conditions by oxidizing the iron atoms in hemoglobin, and by reducing oxygen supply to the blood stream and tissue. This condition is called Methemoglobinemia; closely related to the effects of DCS.
Source? Reference? What is DCS?

Anyway, what's fishy is your reading skill, but you are talking about nitrite not nitrate. BIG OOPS! Brown blood disease, methaemoglobinemia, is caused by nitrite poisoning, sir.

NITRITE - HIGHLY TOXIC TO FISH
NITRATE - EXTREMELY LOW TOXICITY TO FISH, IN THE CASE OF TILAPIA NO KNOWN TOXICITY HAS EVER BEEN OBSERVERD TO MY KNOWLEDGE (which you are free to question)

I have for a long time thought that nitrate had some toxicity, but my knowledge was based mostly off forums and information given to me by hobbiest aquarium pet keeper types. They like to maintain the concept of nitrate toxicity in their folklore, which is probably unwittingly reinforced by pet store salespeople. I now seek my answers in scientific literature, which is far superior in both accuracy and consistency.