For the future, there are a lot of things to look for/at when you're looking for a labored breathing or stressed ray. The spiracles(holes behind the eyes) will become whiter, raw and look worn if the water is bad - the gill area and stomach can also turn pinkish red(not to be confused with the standard pinkish belly some juveniles have). The whole body will lurch up and down with heavy/labored breathing and you'll see them appear to be almost bouncing if it's bad enough.