Filter intake measurement conversion

[Zoe]

Hi!

I have an eheim canister filter (2213 I think) which uses 1/2" hose.  I'm looking into some aftermarket attachments (namely, surface skimmer and pre-filter) on ebay and they come in various sizes, but in millimeters: 10/13/17mm. Which one is for 1/2" hose? I am thinking it's 13mm but I wanna be sure.

Thanks!

Zoe

[daworldisblack]

12-13mm for 2213