The wall thickness of the plastic tank is a thicker than the metal tank. While the outside diameter is the same, the inner diameter is a bit smaller. The threads are cut into the thickness of the wall which makes it thinner where the threads are.
My plastic tank measures about 21mm, though it did come from a source other than FastTech.
I have another Aqua (red ring / $20) and two plastic tanks on order from FastTech.