I had the same issue with one of my Bluegills. Changed the water, put him back into the tank, then he wouldn't stay upright, so I tried to hold him upright, moved him back in forth trying to get the water flowing through his gills, held him over by the filter (Had oxygen blowing out of it) he began to lose his color, and soon enough I lost him... He would do a strange gasp for air when I took him out if the water (Thought he was dead a few times)
I still have no idea what happened. I don't think you'll have much luck... but I do hope it goes alright. Good luck.