You are correct SZatek. Boiling water raises the level of all contaminants by removing pure water and leaving behind the minerals. Finding nitrates in a water supply is really not unusual. We mainly use the nitrate level to tell us how much ammonia has been processed. The standard, more or less, is to do water changes so the tank water never exceeds the levels in the tap water by more than about 20 ppm. That means we are doing them often enough to prevent the build up of most contaminants, it does not give a particular safe level for nitrates.
JoshuaA, since only men are ever color blind, I am sure Szatek is right on that point that "on average" women are better at distinguishing colors than men, no matter where he read it.