"Aqua" empowered us with this mighty blow
against the Patriarchy:

I've followed the basic idea -
Flow input to a vaccum from a gravel cleaner -
bio balls
sponge at the bottom.
1 bubble per second

Problem is the canister fills with co2 - like an air pocket of Co2 -

Is this because there is not enough water flow or ?

First I thought of Doing It Myself. But later I found a cheap internal
filter DJ50 (US $12.00).
It has a airline inlet which is for mixing air with water. I am injecting
CO2 in it.

FWIW, I was doing the same thing but found that I got greater CO2
saturation by running the CO2 line into the strainer at the bottom of the
DJ50 (versus connecting it to the airline inlet).

I also tried redirecting some of the output back to the input by cutting a
notch out of the side of a plastic cup and fitting the cup, upside down,
over the DJ50 ... that was also an improvement but looked ugly.

Of course none of this matters if you're getting enough CO2 with your
setup. ;-) Just an FYI for others who might try the DJ50.