What is wrong with my neon tetra, very swollen (eyes and body)?

My neon tetra is one of 4 in a 54ltr tank with 5 harlequins, 4 phantom tetra, 1 rummynose, 2 guppies, and a hillstream loach. At first the neon just looked fat, (thought it was pregnant), after a week or 2 it started hanging around at the side of the filter in the corner of the tank, also bobbing a little bit nose down. This gradually got worse and is now extremley fat and its eyes are popping a lot. Its appetite is completley fine .... HELP!