I'm not familiar with the part in question, but to reduce from 30mm to 20mm you probably need a barbed reducer like in the pic below
These are available from various sources such as plumbing suppliers, boat chandlers, agricultural irrigation suppliers, hardware stores, industrial supply houses etc.
You can get them in plastic, brass, anodised aluminum, steel and stainless steel.
You may not find them in metric sizing, but 3/4" x 1 1/4" will be fine.