Are you going to use a separator bottle? (one that has the lining from the 2 liter bottles connected to it, and this separator bottle connects to the actual tank)
I'm still trying to figure the exact specifics out, but it looks like in the seperator bottle, there is water in it and the air lining from the 2 liters goes into it. This will create bubbles in the water and can double as a bubble counter.
see pic in link
How many DIY CO2 bottles for a tank....