Mine used to do that occasionally. I put it down to too much food* as they'd suck up food at the surface then go down to the bottom and do the same there. Right little piggies. I also thought it could be trapped air sucked in while feeding at the surface.
I still have one, the other died after I'd had him for about 12 years or thereabouts, so I guess it's not too serious (if that's what your problem is of course!).
*Too much food in the sense that they were temporarily bloated rather than overweight.