We've had threads on this before but I did it again tonight - back from the brink for fishy no 3 - so thought I'd post a reminder.
If you have a goldfish going belly-up all the time, or else desperately swimming downwards to stop itself going belly-up, give it a bit of pea!
One frozen pea, thawed, with skin removed and cut into tiny pieces, dropped into the water - it will sink like a stone but even a floaty fish should be able to propel itself downwards enough to pick a piece up off the bottom. If it really can't manage it, fish it out (sorry ) and put it in a pudding basin with some tank water, then drop the pea into that until it's got its equilibrium back. Quite magic.
Signed: Fish Doctor Ursula