If there are no other symptoms and no other fish are sick, then it could be swim bladder disease or that the fish has gulped in air (while eating at the surface or due to poor water quality). From the book I have, there's not much you can do about it if it's swim bladder problems. If it's from gulping air, during feeding, hold the food under water before releasing it, so they have to eat mid water; or gasping due to bad water quality, fix the water quality.
The experts here will likely want to know the following:
How big is the tank; how many of what kind of fish; what are your ammonia, nitrite and nitrate readings; what is your maintenance schedule / routine?
Liz