I used 4.5" OD tubing with 1/8" wall thickness. The project that I referenced didn't require any type of acrylic bonding - 4.5" fits directly into 4" PVC fittings. And I could get it cheap at
USPlastic.
I also liked the
plugs used in the project- tight fit without worrying about unscrewing 30 screwsand all plastic. Also a drop in fit to the 4" PVC. Lastly, be sure you order some
clear pvc if you order from them, for a bubble counter.
I don't know how well this one works yet. I've only finished it, filled it up with water, and let it run for a few weeks now. I still need to get a regulator, CO2 tank, and peristaltic pump...