Sorry gang. . I was remembering the individual API Nitrate test with the "pills." Good point about Nitrogen run off. Even in Chicago, our Lake water has a certain amount of Nitrate straight out of the tap. I have not personally had to use the Nitrate removers yet, but I think that time is nearing. . .
Yes. If nitrates are high and regular water changes have not been done, it is essential to lower it, but do not try to do it as quickly as possible with one huge water change.
I would do daily or even twice daily small water changes of maybe 5-10%, gradually increasing the percentage changed. Many people who hope to remedy an algae problem or nitrate problem fail to realize that a long-neglected tank can have water very different in chemistry than what is likely to replace it. Over time, the water degrades and may become softer and less acidic, resulting in a pH shock to the fish when the owner does start changing the water again.
So go slowly and steadily at first monitoring the nitrate with your test kit. Increase to a volume that is reasonable for the tank size and stocking level you have, and then be as faithful as you possibly can to keeping the maintenance schedule.
re: folklore: look at any ill-kept oscar or discuss if you don't think excess nitrate does damage. HLLE is highly associated with poor maintenance and high nitrate.