fish swimming funny

[x_tra]

yesterday i noticed that one of my serpae tetras (i have six)is swimming a little funny.... he swims with his head pointing up at a diaognal but not straight up. he seems healthy other wise(good color, eating, no one has been picking on him.)and when he gets frightened he swims normal......
but i was just wondering if this is maybe some kind of internal pairasite or some thing like that?

 

[Allivymar]

He may be having swim bladder issues (the swim bladder helps the fish remain upright). Theres a number of causes; digestive, disease, toxins, injury. Its hard to say an exact cause without more information; are there any other symptoms, or have there been any changes recently? Also, what are the fish fed?