Without the swollen body I would say estuary has it pegged. Popeye is more of a result and not a disease, as stated before commonly caused by poor water conditions and internal bacteria infections. I would just go with lots of water changes to keep nitrates around 10ppm and add 1 tablespoon of salt per 5 gallons of water. Up the temp to 85 degrees and add and airstone for extra oxygen. If you don't see improvement in a couple weeks then go for a medication. Not sure about the stingrays though, may want to post in the stingray forum.
