Hang in there brother... all ray keepers will experience this. Did she eat the pleco's stomach contents? Stress plays a much bigger part than we know... I've had rays do fine in the morning and dead by the afternoon for no clear reason.
vincentwugwg i know bro... not my first stingray death but definitely tragic. This boy is about two years old and has always been in excellent shape. Always begging for food and was the dominant stingray in the tank. When I netted the pleco out, only the tail was chewed up but the stomach contents inside already turned quite dark. Just like how people have mentioned, he might have choked on a stinger that was shed from the other two stingrays or even his own. I've always doubted this as it's never happened and I've picked up at least 10 stingers in there or more. All in all though stingrays are such interesting creatures.. I don't think I'll keep high end BDs etc until I get a real proper set up like yours. I'm already surprised how well they did for my newbie setup for this long.
So when you put the hose back in the tank do you dose with Dechlorinator?
China uses high levels of .Chlorine and Chloramine plus NDMA according to google. Chlorine and Chloramine vary in strength quite radically from time to time depending on things like bacterial outbreaks and a dose of chlorine can kill a fish easily (chloramine is far worse and normal dechlorinator cannot remove it), one fish may be weaker or more likely to suffer than another so you cannot rely on the fact that not all fish died.