Nitrates aren't extremely toxic to these guys unless you have massive amounts. I don't think theres a magical number, do you have an anemone and high nitrates?
It actually appears that anemones like nitrates to a certain extent. The zooxanthellae they harbour seems to eat this product in limited quantities. They appear to tolerate higher levels than most LPS and SPS corals. Some may even appreciate it.
However,the toxicicity of nitrates has not been agreed upon yet. But you can come to the conclusion the less the better.A high salinity and ample water movement would be a better goal.