http://www.youtube.com/watch?v=G7g9NarTvpE&search=trained+japanese+goldfish
My favourite fish reference
But back on topic a touch... I think it's a good idea to get a broad range of reference - I mean, don't just use Nemo as your reference and stylistic goal. (You'll only end up very frustrated and disapointed!) Check out the BBC motion gallery and get a good all round idea of how fish move (yeah yeah... we all know how a fish moves... but you're going to need to really study it

) Then when it comes to sketching out ideas - think about the fish's personality while you are sketching.