Just an update: I am using a cigerette butt on the end of CO2 line and it is making a very fine stream of bubbles. I placed the pipe end at the substrate level just below the filter outlet. The filter outlet pushes the bubbles all around the tank! I don't know about the efficiency, but my Ejeria Najas has started sprouting very dense leaf clumps, and the leaves are a bit red.
Also, I still have the venturi pipe at the filter outlet; I just keep the air line under water during day, and pull it up during night.