thx all for your replies.
@lazyman: I already tried lots of aquariumshops and didn't find anything. And the tubing I use is tygon with 18mm outside thread and those quick-connect couplings will be WAY too big for that. I had the same idea, so I checked already.
@Jort: In the gamma here in Ieper they don't have them, but if you go to your local gamma and you'd be so kind to have a look, I'd be very grateful. Inside diameter is 13mm.
@biCker: I really need them in 13mm. thx anyway
@Xploited Titan: looks like I'll have no other choice than to go with these. But on most shops where you can rate the product, they don't get too much points. Also I like the others and they are the same style of the Y-adapter with my dangerden rbx.