You need a lot more flow through the CO2 system if it's an external method.
You can also use a mazzii venturi valve.
That will require about 400GPH or so, and you need at least that much pushing the CO2 around.

For a 120 gallon tank 600GPH's of canister filtering is needed.
With some of the larger filters, you may be able to drive a venturi.
If not, use the disc and place near the out flow of the filter blasting the mist downward and the length of the of the tank.

Regards,
Tom Barr