Help with my Skimmer

[Eb0la11]

Hey guys, just plumbing up my skimmer and ran into a bit of a question mark when putting my skimmer together. Its a used unit so I dont have instructions or anything. It works quite well, doesnt leak and is doing the job.

What I didnt understand is I have this big like 3 inch o-ring that sort of fits between my collection cup and the top of the unit, but not quite. When its in there I cant drop the cup down into the grooves and twist it tight onto the unit.

Is this the right place for this o-ring? Does anyone else have an o-ring in this spot?

What product can I put on the o-ring to lubricate it thats reef safe? (dont answer only this question, cause Im pretty sure vasoline would work and is fine, the other questions are more important.)