A little more info could help:
Any other fish suffering/showing signs of being sick?
Water parameters?
Added any new fish lately?
It sounds like Swim Bladder, which can be caused due to a "blockage" or baterial infection. Have you seen the clown poop lately? If not, you could try taking a pea (yes a green pea) dicing it up and see if the fish will eat it. This will help to relieve any blockage since it is a natural laxative. If the fish starts to return to normal, then keep with the pea until he's healthy again and able to swim normally.
Also, if there are other fish in the system, keep an eye on them.