Basically you need to make sure what scale is being used when an author writes that X level of ammonia, nitrite or nitrate is given.
For a fishless cycle the researcher will almost always use two measure TAN (total ammonia nitrogen) expressed as ammonia-n. This may also be given as NH3-N. It is up to us as readers to pay attention and know which.
For example, Dr, Hovanec always states the the number as -n. So when he states one should never allow ammonia or nitrite to exceed 5 ppm, he makes it clear this is ammonia-n and nitrite-n. But most members here tend to use the API kit which does not measure on that scale. So if you want to follow Dr. ovanec's advice, you need to adjust either his numbers to be equivalent to those on the API kit or else adjust your test results to be equivalent to the scale he is using.
Now this is fine when it comes to ammonia. His 5ppm is about 6.5 ppm on the API kit and that kit will measure up to 8 ppm total ammonia ions. We run into problems with the nitrite kit. 5 ppm of nitrite-n translates into roughly 16.5 ppm on the total ion scale. And the API kit stops a 5 ppm.
Nitrate is a less clear situation as the research is more concentrated on ammonia and nitrite than on nitrate. But this may give you a bit of an idea:
The acute toxicity of unionized ammonia; nitrite and nitrate to the Indian major carp Catla catla (Hamilton) was determined using static and continuous flow through systems for 24 hours. The median lethal concentration (LC50) values for 24 h of ammonia (NH3-N), nitrite (NO2-N) and nitrate (NO3-N) were 0.045 mg/l, 120.84 mg/l and 1565.43 mg/l in static test respectively and were 0.036 mg/l, 117.43 mg/l and 1484.08 mg/l in continuous flow through test respectively.
from
http
/www.ncbi.nlm.nih.gov/pubmed/12602850
Now I realize this is a study of mortality, but it does give a good idea of how different the levels are are for each of the 3 nitrogen compounds. Not all the levels are given in -n scale. What one can take away from the study isn't any absolute rule for all fish but rather an understanding of the differences:
Ammonia-n at .045 ppm of NH3 killed 1/2 the fish in 24 hours.
Nitrite-n had to be at 120.84 ppm to do the same damage. That is 463 times the NH3 level..
Nitrate-n had to be at 1,565 ppm to do the same thing. That is almost 35,000 tines the ammonia concentration and 13 times the nitrite level.
If you are really curious, do a Google Scholar search for "toxicity of nitrate to fish"
Incidentally, your comments support the idea that one should always plant when cycling a tank at the outset. Because plants eat ammonia they will handle a certain portion of the normal cycling chores. This means one needs a smaller amount of bacteria to deal with ammonia etc. If one cycles a planted tank before the planting one is creating more bacteria than needed long term and thus also taking way more time to cycle than is needed as well. Read on Tom Barr's site and the folks there laugh at people who fishlessly cycle well planted tanks.
One last comment. In the states drinking water is required to contain not more than 10 ppm of Nitrate ( I do not know on which scale). However, this doesn't mean that will be what comes out of one's tap. Private pipes are not regulated- i.e. once the public water supply is connected to the homeowners pipes, the rules do not apply. In those private pipes it is very likely there are nirtifying bacterial colonies. These thrive on the ammonia caused by the breakdown of chloramines used in the treatment plant. Abd as we all know the end of nitrification in the absence of the final stage is nitrate.And this is the biggest cause of nitrate in our tap water.