Great mindes think alike... lol.
I also use a diffuser bell that is located right under my aquaclear 500's intake tube. When the bell is full, the c02 bubbles get sucked right into my intake. I don't have a c02 test kit, so I don't know what the ppm is, but without using a comercialy bought diffuser, I belive that this is the best way of diffusing c02 into my tank. I wish I hade a c02 test kit so I could actually know how efficently the c02 is being diffused. I use a DIY c02 generator. I connected two, 4 liter milk jugs with sillicon air toobing and a 2 way gang valve. I thought that having two seperat jugs would double the pressure of the c02, but I'm only getting about 1 bubble every 2 seconds.