Check for ammonia, nitrite, ph and then nitrate and make sure you have maximum surface agitation as the oxygen may be low.
How old is the tank and filter, how long have you had the ray?
Some rays do just do this sort of thing but it is rare, normally this is a water quality problem.
I would do a large water change and do not use dip strip tests, buy a new set of kits as they go out of date.