I think I'm confused. NO3 is a compound commonly referred to as Nitrate. Nitrate is not Ammonium which is NH4. As I understand it, once ammonium becomes NO3, a chemical reaction has occurred, making it no longer NH4. Correct me if I'm wrong here, but NO3 cannot also be NH4, even if NH4 was the original source. (This is not considering the impact of possible harmful bacteria build up and other compounds in the waste products that would not be present in chemically pure nitrogen addition through fertilizers, including NH4.)
I agree that NH4 (as well as NO2) is much more lethal (with much swifter results) than NO3. This does not make NO3 in a high concentration benign. If we were only talking 40ppm, I would be inclined to say that the concentration of NO3 is on the high end of a safe range.
The starting point is critical here.
NO3 from KNO3 is VERY different than it is from NH4 as the source.
We can dose NH4 as NH4Cl or as say fish waste or as rotting plants etc.
We can dose NO3 as KNO3.
This has caused many myths concerning NO3 in planted tanks, since we are typically the only sub hobby group that doses KNO3. NO3 as the end product of organic N(fish waste mostly) with NH4 and NO2 as intermediates and toxic, but NO3 itself is relatively benign.
You'll note the most sensitive, like coldwater trout fry etc are sensitive if you do any research, but warm water tropical fish are at several hundred ppm's. Florida and Asian fish farmers know this.
So in the practical aquaculture side as well as the scant research available on warm tropical species, as well as a simple test to see myself and the mistakes of others.........this is not the case.
Dose and see if you can kill your livestock with KNO3 if you feel I am incorrect, then try using NH4Cl or (NH4)2 SO4.
Note, even if you do kill the livestock, this does not imply there may have been other factors involved, but if you add say 100 ppm and nothing is harmed, what does that imply? That 100 ppm is NOT lethal or harmful.
We can and many planted folks already have, over a decade + time frames, shown this to be the case.
Much easier to prove what something is NOT, than what is. Once done, you move on and do not cling to the myths. You look for other more interesting possible causes to falsify.
If you can show that fish are stressed by 40 ppm or more as KNO3, why don't many many other planted hobbyists also not report this? Why do many report 100ppm + is multiple cases using KNO3 errors in their dosing with no ill effects?
There are 101 ways to kill/stress fish that have nothing to do with plants fertilizers etc, how can you rule out those possible causes with an aquarium or two? You cannot, but you can rule what things are NOT causing the fish issues.
So, my question to you, have you personally tested this to see if you can harm or hurt fish using KNO3? If so, what ppm's and what livestock was used?
I've done this with many species and Discus, and shrimp, shrimp are even better because they are easy to breed and we often have plenty of culls to work with, they can live in small tanks etc, so many replicates can be done.
Now try this with NH4Cl over a range say 0.1 to 5.0 ppm.
A full 100-1000x less concentrational difference, I'd say the KNO3 source NO3 is pretty harmless by comparison.