In order to fit smaller size hosing (18mm), adjust the tap as follows:
1.) Image 1 below depicts the tap from the box with additional adjustment piece.
2.) Unscrew top fitting as seen in image 2 below.
3.) Remove top fitting and store with aquarium equipment spare parts in case of future use.
4.) Unscrew longer nozzle.
5.) Remove the piece unscrewed in step 4; this will expose the o-ring.
6.) Use the smaller adjustment piece to fit the smaller hosing size.