I was just wondering where your KH is?
You can measure the CO2 if you know the KH and the PH of the water column.
If you crashed the PH overnight, its important to know the KH of your water. The KH will act as a buffer limiting the change in PH for the CO2 added.
I think hagen makes a $10 membrane diffuser that would be good for you tank. On DIY systems airstones are of limited use. The pressure from the yeast bottle is not usually high enough to get the fine bubbles out of it.