In drinking (tap or bottled) water high nitrates are a concern for infants and many animals. They have certain bacteria in their digestive tract that convert NO3 to NO2, so causing methemoglobinemia. AKA Brown Blood Disease.
I do not know how they decided that 10 ppm NO3 was safe in the water, but the goal was to protect infants.
Adults may be fine drinking that water. I would do more research, though, and probably do something about the water. RO, bottled, or something.
If there are nitrites in the aquarium water, fish can get this, too. In the aquarium, NO2 from the water crosses the gills and enters the blood. This makes the blood not carry oxygen very well.