Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
If the airline is going up (or to a lesser degree sideways), when the CO2 reaches the end of the line, the first bubble that comes out sets up a chain reaction, where the suction of the filter draws more out. The longer the portion of the airline going up, the worse it is.
If the airline is always going down, that doesn't happen. The angled cut just directs the bubbles into the intake, otherwise the filter might not draw them all in.
There's probably other ways of accomplishing the same thing as well, this is just how I do it.