Yes the induction of Co2 as opposed to air is an entirely different matter as well. 1/4 " line is fine all the way up to the last orifice into the venturi. It allows you to offset the .036 opening to the leading edge. This increases the velocity of the gas flow when using power heads at a 150 to 180 gph flow rate. A very small augmentation to the gas flow produces a much finer mist, and allows you to maintain what precious little flow you have ( The flow rate is perfect for the reactor, but has a low static pressure as it relates to venturi performance).

I use this intake in a recombinant fashion to purge the reactor. Your original design was more than adequate for most systems. The short comings were always in the powerheads themselves. A longer reactor tube can handle the increased flow rate, but the escaping mist is actually a perk in my book. Now if you modify an internal duct within the reactor tube (Mixing Condenser) you can compress the stream for a more efficient exchange of gas, but it's really a mute point as the reactor is quite efficient already. Not many people push more than 180 bpm.

Venturi injection for Co2 is not really tricky, but it's considerably different than o2 or o3, and freshwater doesn't enjoy the same luxury of density as saltwater. Prof M

Your method of tapering the 3/16" pipette is much simpler I just have to figure out how to orient it within the venturi orifice ? A proud edge to the orifice would increase the vacumm energy !