For bottom drain application 3″ and larger is recommended. 2″ could be used as a mid level return to bypass the waterfall for those of you in colder climates, thus preventing super cooling your water.
Need help installing your bottom drain bulkhead? (Find out more here)
Installing an Air diffuser bottom drain? You would need a bulkhead for the air line. (Find out more here)
michael gamage (verified owner) –