it's got a swim bladder infection most likely. i saw this happen with a ryukin goldfish of ours, it progressively got worse till he just couldn't swim right at all, could barely eat or breathe, and had to be put down :(
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)