Rubix,
It is 3/4 inch PVC tubing.
Once i had all the T pieces and 90 degree bends in place across the width, they all butted up together and just fit in the tank.
if you look at Martin's 'exploded' manifold here
http://www.loaches.com/articles/river-t ... old-design
then you will see the the spacers in between the T's.
My tank is 12" wide and with no spacers (actually very short and not visible when constructed) the manifold is the perfect width for the tank
Hope this makes sense and helps.
TC
PS, still waiting for the pH to drop! (definately think cement background is to blame!)