Fluval fx5 setup

I'm setting up my new Fluval FX5 filter that I bought used thru Kijiji (like Craigslist), and I'm paranoid that I'm missing something. I see one small o-ring on a white plastic piece that sits inside the lid. Are there any other small o-rings I should have? I tried researching online but none of the Fluval parts lists or assembly videos I've come across even mention the small o-rings (or the white plastic piece).

I just want to make sure I have all the parts I need, so I don't have a major leak.

thanks!