I use a wooden chopstick, just break off the tip piece & stick it in the the tubing, then put it by the intake of the filter.
Oh & a lot of people complain about a white fuzz that starts to get at the end of the chopstick or diy diffuser with diy c02...... I used to have that problem But then What I did was run the airline tubing to a hard plastic tubing, then into the chopstick..now No problems of buildup what so ever, been going off the same batch of c02 for 1 1/2 months now
Yes i had that white stuff on my sillicone tubing as well. I've just tried putting it into my HOB filter and the blade is chopping up to tiny bubbles. This method looks pretty good to me.