I go with the comment above. I have a 304. The flow path within the filter is not simple (as in Eheim filters). The intake runs down the foam pre-filter, along the bottom, and up through the pads and filter media of 3 baskets. I have looked at the 304 filter during periodic cleanings and I see evidence that intake water is missing out some of the modules. It is a clever, somewhat convoluted system. Not nearly as simple as the Eheim 2224, for example. I run CO2 into an Eheim 2224 with no problems at all. I would be cautious about running CO2 into the Fluval 304 because of the long flow path and possibility of gas locks.
When running CO2 into the intake of an Eheim 2224, the bubbles generally dissolve before they reach the cannister (along the 4' hose).
If the tank is small, Hagen Ladders work well.