Aaah. That was what I thought and why a tap water level of 4 to 16 bothered me.
I think it's important to understand what those numbers mean in reality.
<20mgl = optimal conditions
prolonged exposure of 20mg - 80 mg = possible impact on health issues, growth rate, mortality rate, etc. It means that some of the fish might live slightly shorter, some fish might develop chronic illness and some fish might die. But percentage is quite low (between 1% and 10% of population) compared to fish in optimal conditions.
>80mg prolonged exposure = you starting to see increase in mortality rate, serious adverse health effects, fry survival and growth rate, etc. But still percentage of population.
>200mgl prolonged exposure = and you start seeing serious increase in percentage of population that has seriously adverse health effects, mortality rate, etc.
>400mgl prolonged exposure = serious increase in mortality rate and poisoning causing serious deformations, irreversable damage, etc.
It's worth noting that some fish species are a lot more sensitive than others to nitrate poisoning (Discus for one) than others.
And that fish can aclimate to Nitrate levels but new fish can get Nitrate shock poisoning from much lower concentrations than fish in the tank with elevated nitrate; resulting in nitrate shock death.
That's my understanding of several studies I read on Nitrate poisoning.
In conclusion, how I see it:
20mgl rule is the same as humans living by all health advises of what might cause illness or death (which is pretty much everything)
<80mgl is same as human not eating particularly healthy diet, having few drinks here and there and living in the city with not so great pollution levels.
Chances are you might live a year or two shorter or have 10% more chance of being ill last few years of your life than someone from 20mgl rule.
80mgl> you are eating excessively and unhealthy food, are overweight, don't do much physical activity, smoke and drink bit to much.
200mg = you are really overeating on sugar, fast food, heavy smoker and alcohol drinker, drink 2lt of Coke a day and spend all your time in front of the TV with beer in one hand and smoke in other.
400mgl = You are drug addict and will eventually overdose if you don't die from heart attack or something else sooner.
I think most fish owners live much much unhealthier lives than what they stress about with their fish.
But that's just my opinion on the mater.
GL