I am freshwater ray specialist and have kept them for well over 10 years. If it is a standard potamotrygons (usually sold as reticulated stingray) then this is fine. All Pota rays are more active during the nightime rather than the day as this is their nature. I shouldnt worry.
As for water quality, if she or he is wild caught and acclimatised, then dont worry. Everybody will tell you that its bad to keep them in water that is crap, and I agree for the long term, but standard peru rays are hardy creatures and will do fine (FOR THE SHORT TIME..)
For their characteristics, they only feed at night., there are other classifications that are more boistrous, such as marbles, tigers, and leopoldi, but standard rays are only nocturnal hunters. Best fed on bloodworm if you ask me. Its cative bred, vitamin enriched, and easy to eat. Also, wha tthey dont eat, burrows, they hunt and it addds to the natural behavour... Id do this if I were you. Its also cheap...
One thing. the 125g... Whats the dimensions of this please. Dont mean to be a sour puss, but if its a thin tank, (IE under 18" in width) youll run into problems...
In any case, if I can be of any help, message me, or post here, and ill help.
"Mother nature never intended us to keep Reefs. Just because its for sale, doesnt mean you buy it. Research, learn and make an informed decision, not an impulse one........."
28g Ornate Seahorse Reef (Planning stage)
42g Warty Frogfish Species Tank (Planning stage)
125g Pacific Reef Setup (Underway)
32g Planted Freshwater Community (Underway)