I agree.. Nitrate is a good indicator of the waters age (though it depends on the quantity of food being consumed and so on...). But im still not sure if rays are more or less nitrate sensitive than "normal" fish. On praticafishkeeping.com a ray expert, Richard Hardwick, states the following:
"Some books suggest rays can tolerate NO3 levels of 100pm or above. Nonsense. In my experience, levels of just over 50ppm for long periods can be detrimental to the health of these fish, so regular maintenance is of paramount importance"
Its far from the 5 ppm.. But still.. What do you guys think..?