I would assume chloramine would kill all but maybe a weak ray could succumb more than others.
You need to do a complete work up of the water for Nitrite, Ammonia, nitrate, ph, hardness and maybe copper. Use brand new kits and not dip strips.
The water change may be throwing you off what happened. After all of you eat a curry for the first time in your life one day and die of a heart attack the next it doesn't have anything to do with the curry.
What type of ray, my guess is wild caught retic....?