Nothing in this thread seems to pass along the usual lore that I've heard mentioned to beginners. Catpants, the wording of your original question implies a concern that nitrate(NO3) is like ammonia or nitrite but just has a higher ppm level at which things are ok, instead of zero.
From the discussions I've been in on TFF and older threads I've read here I don't think that's the way to think about nitrate. First of all, there have been some interesting findings discussed here in the past that nitrate levels as high as 400ppm or even 1000ppm have been found to not harm some species, the point being that nitrate is generally not toxic in the same sense as ammonia or nitrite. I believe the extent to which nitrate might be harmful, however, varies greatly by species.
As a testing tool, nitrate(NO3) level is often used to gauge the extent to which a tank is maintained. Say, for the moment, that we have no live plants. A tank receiving more frequent gravel cleanings and water changes might be found to maintain a nitrate level only 5 or 10ppm above whatever the source water level is. So if the tap water tested at 10ppm, then the tank might consistently measure at about 20ppm. If the tank has heavier waste producers and/or is somewhat less maintained, we might see it sustaining 20 or 30ppm above the source water level.
Traditionally, one use of nitrate tests has been to serve as a gauge of the maintenance level in this sense when maintenance habits are unknown or for the aquarist to monitor his/her own maintenance results when the habits -are- known.
Another function (well, a further aspect of the above function) of nitrate(NO3) tests, as I understand it, has been that they can serve as a "canary in the coal mine" sort of indicator. Its understandably easy for beginners, given all the complexity they are faced with, might feel that tank water is "just plain water" with 3 things to worry about, ammonia, nitrite(NO3) and nitrate(NO3). Unfortunately, nothing could be farther from the truth! Tank water has, probably, most of the elements of the periodic chart and hundreds of inorganic and organic compounds varying in it. Most of the things in tank water are just to complicated or expensive for us to test for. To some extent, our simple tests are like searching for the dropped object under the streetlamp. In the case of ammonia and nitrite, they warn of primary dangers, but for nitrate its really more of a case that this particular indicator chemical is cheap to test and we can do it. If we could afford to test other more complicated things and fully understood their importance, we might very well be doing that.
Also, I'll make a final comment that this all plays into the beauty of regular water changes. My rule of thumb with fertilizers is that things like NO3 due to those are not things to worry about, especially as long as reasonable water change frequency is maintained.
~~waterdrop~~
