Yes rays are sensitive but not that sensitive IMHO. 0.25 ammonia will not kill your ray. Ive put rays in freshly started tanks and never had any trouble with that. Id say that its stress that killed this ray. From what i understood he recieved the ray and put it in one tank and one or two days after that moved it to a new tank.
I didn't move the ray I made big changes in aquarium , light on sometimes , not enough air circulation for small pupp that's was my mistake
I got to know from one of breeder from Facebook where I post this rays previous owner have feed him before travel and that creates more ammonia in water and while traveling stress he **** and piss a lot so he got poison my water parameters was not big issue. From whom I took ray he is just fish keeper like me so he don't have any idea not to feed any ray before traveling.
It was bad luck as well as my mistakes.