My betta fish body is jerking - I think he is ill

[Tee]

I have a male betta with a healthy appetite. I change his water weekly and he is fed once a day. I've had him for about 3 months now but recently (say 2 weeks ago) I noticed that his body was jerking - has anyone experienced this before? if so what is the problem? how do I correct it? Thanks.

[Flashygrrl]

First, standard questions:

Tank size?
Water temp?

Also, is it the whole body jerking or just part of it?