I would have been surprised if it was anything but zero.
It is possible that there is some nutrient build up in some small blind and stagnant irrigation canals that snake around fruit groves, during the hot season.
Otherwise, pretty much every waterway in Thailand would have zero nutrients.
Back to the original question, considering where NTT come from, keeping them in a tank with any buildup of nitrates is the usual reason for cloudy eyes, as they are all wild caught and have absolutely no tolerance.