I have made my own rocks for inside the tank. When I made them I ran loc-line through the inside of them so I can place a good current of flow directly on the corals. I was looking for a mainfold so I can run 2 rocks off of one pump. It needs to have a barb for 1/2 inch tubing inlet and 2 1/4 inch barbs outlet I already have the barbs. Just looking for the manifold. I think I will just do the same thing with 2 reducer "T's" instead.
Have you looked in the irrigation section of your local Large Hardware Store? I don't recollect seeing them in 1/2", but... <shrug> Actually, the reducer T's seem a little more straightforward to me.