1- Aros getting stressed, mostly when young, when lights go off is pwrfectly normal.
2- your Aro is not stressed. It is not afraid, it is not hiding, it is eating pellets, for chrissakes.
3 - raise your hands to the skies. You have a mellow, glassbanging, Silver ( i have. 2 footer jat like that. A real wet pet. I can hold her inside the tank, shove her this way and that way, with no problem whatsoever.
Good luck. Appreciate that critter and leave worrying for what needs worrying.
On another note, nitrates can never be zero. You must have been referring to nitrites.
M