Is it possible to use a canister type filter for the diffuser of a CO2 system?

Yes it's possible and practiced quite often. No modification needed.

Simply take your CO2 tubing and direct it through the intake strainer. The bubbles will be sucked into the filter and slowly dissolve as it goes through the media.

-John N.

You could run an in-line reactor as another option, but for DIY CO2, it might not be the best route to go.

You might not want to do this. CO2 and water makes a mild acid. If your flow rate is good enough and the acid does not touch your seals it should be ok but it is known to desolve certain rubber compounds. As an example of its powers I use a simple diffused and it desolves one airstone per month. I run CO2 though all silicone tubing. How much danger your seals are in depends on the amount of CO2, the flow rate and of what the seals are made. An alternative is to run the CO2 to a downwardly directed output to a powerhead or canister filter, any flow really that will mix the water and direct the flow downward. In this manner the CO2 comes in after the seals. I suppose I am a bit of a worry wart but I don't want a floor full of water.

Brett

What I do is use a Hagen CO2 ladder placed well beneath the output of the filter. The bubbles collect at the top and when they finally release and go upward, they are caught in the downward current and break up into tiny ones. It's sort of one last final mix for after they leave the ladder, which serves to keep them submerged for a few extra seconds.

You can also use an airstone and place it far below the filter output. That way the small bubbles take some time to rise (mixing the gas with the water) and then reach the output where they are further dispersed.