NASA Tang? Isn't that the orange stuff the astronauts drink?
Sorory, couldn't resist...
There are many different possibilities, bacterial infection, problem with the usual tang "gut fauna", blocked passageway, or just in need of a good bicarbonate of soda to relieve gas..
I too would recommend you bring the tank back to normal salinity until the fish gets back to normal. It's very possible that for some reason, the hyposalinity has created a problem in the digestive system, and has things "plugged up"...or created some other problem...
Of course, it COULD be all those McAlgae burgers with french fried xenia you've been feeding the poor little fella...