Hi,
Please keep in mind that there will be LESS mist the further away from the outlet the source of the mist is.............
c02 has more time to dissolve in water the longer the time spent in the pipes...thinner pipe will flow LESS volume of water but faster than wider pipe.
The closer to the outlet that the mist originates, the more will make it to the tank.
Maknwar,
If you step up to a 100x just for c02, may I suggest the use of a mazzei and bypass assembly so you can use some of the extra power from the pump for circulation that is unused by the venturi?
I think you could do very well with a mag 5 or 7 and a DIY PVC reactor. Or maybe one small pump/diffusor on each side? Then a koralie 3 or 4 to help mix it around...
Are you running a sump or wet/dry or closed canister?
Hope this helps.