Whether nitrates are harmfull depends on the fish species, most common community tropicals will appear fine even when kept in water with nitrates over 100ppm, this does not however mean they are thriving and colours may not be as bright as they should be, they breed less frequently, have fewer offspring and shorter lifespans. Try keeping more delicate species like wild or F1 discus, stingrays, Pimeloid catfish, rarer L numbers or many of the wild caught Characins and oddballs like this and pretty soon you will have dead fish.
The test by scientists on a few marine species does not give conclusive evidence that high nitrates are harmless to ALL fish, just that those tested showed a tollerence, i have personally witnessed the whiskers of Pimeloid catfish curl up and fall off when exposed to nitrates above 80ppm and have noted enormous differences in the colour and vitality of fish when nitrate free R/O water is used for water changes instead of heavily nitrogen polluted tapwater. Ideally everyone should aim to keep their nitrates to under 50ppm or at least within 10ppm of tapwater readings.
And to answer the original question the highest i have tested tapwater at is 70ppm and if i remember rightly the legal European limit is 50ppm. Higher nitrate levels are usually expected during the summer months when water is in short supply and what is available is being put through the treatment plants faster to keep up with demand.
and I just couldn't have said it any better myself.
