How long did you aclimatise the ray for before introducing it to the tank? With rays you should always introduce them to their new water as slowly as you possibly can even if the water tests are all identical, i spent 10 hours acclimatising my ray which came from a store just over a mile away. The best way to do this is with the drip method , place the ray in a poly box and have a length of airline leading from the tank to the box, start a syphon in the airline and tie a loose knot in the end that is not in the tank, then tighten the knot so that the water drips out at about one drop per second. Leave the water dripping into the box containing the ray for as long as you can, removing excess water before it floods over the carpet.