Does anyone use a Fluval FX5?

[AndrewHobday]

bit if a random one, Ive got a Fluval FX5, the motor started making a racket today so took it apart to have a look. When i took the motor off i found a washer/seal/gasket stuck in the impeller, its about the size of a 1 pence piece, made of black plastic and looks like a penny washer. The thing is i have no idea what it is or where it goes? ive looked through the parts list on fluvals web site and cant find anything that looks like it, its a bit of a headache because im afraid to turn it back on incase it leaks or something goes wrong. Does anyone have a clue what it might be or have had the same thing happen to them.
Thanks