CO2 Injection

I have a question and would appreciate it if anyone could help!!!  I
currently have a 55 gal. tank (w/ fish) and i am injecting CO2 with the yeast
method.  I am running two 2 liter pop bottles into a gang valve where i then
have 1 piece of air line tubing running into the tank.  That one piece of
airline tubing is run into the uplift tube of my Whisper 3 power filter.  I
have it constucted this way so the CO2 bubbles will be "chopped up" and will
dissolve into the water easier.  When I first test the water (before the
addition of CO2) the ph was 7.8 (the highest my test kit went up to) and i
can get it to drop down to 7.2 with CO2.  My problem is how can i get my ph
to drop down into the 6.?? range?  Is the way i have my CO2 system setup
Please help!

Chris Simmons
aka...griff1324 at aol_com