So standard air line tubing would suffice I assume?Outside doesn't matter. Inside just depends. As deep as it takes to find the sweet spot.
I currently have a simple CPR internal overflow box with a 1" bulkhead going into a 90 degree elbow, into a ball valve, then down to the sump. It's relatively quiet if I choke off the flow ever so slightly to keep the bulkhead opening submerged. Seems like the Durso method is popular for some reason (better flow and more silent?) Will seeAnything that allows air to pass through it will do.
Is it having trouble draining? A durso won't necessarily make the drain more quiet.