This may sound TOTALY STUPID but i gave a fantail to my father-in-law once (couldn't bare to watch him die, had him too long) with exactly the same symptoms as yours. The fish was dying with no hope, so my father-in-law tried as one last resort of placing a disolvable asprin in his bowl of water, within 24 hours the fish showed improvement, within 48 hours he was eating again and made a full recovery.
We call him Dr Dolittle, the fish is still alive now !!
As i said sounds TOTALY STUPID, but it worked !!
NOT saying you should try this or anything though as i am no expert like the rest of these guys, but you never know.