I assume you don't have a qt tank, but you should probably isolate the fish if possible.
I am no SW
expert, but if the fish was upsidedown I think it is probably terminal. A comet of mine did something similar to this so I filled a cup with cold water, and allowed ice cubes to melt into it. I placed the fish in the cup and it quickly ended his struggle. I am not sure if something similar can be done with salt.
I am not saying you should kill your fish, just listing a possible solution.