Hi Ziggy, I'm by no means a pro but I've researched many ways of incorporating Co2 into a tank, externally. I'm not sure how people tap into the intake line on those filters, but in many instances, it has caused problems. Directly injecting Co2 into a filter will cause burps in the system by constantly injecting air into a filter that is not designed to have air in it all the time. It can cause premature motor failure, or an impeller lock from an overload of air in the system. I've also read the Co2 gas itself is not good for the parts.

If you want to incorporate Co2 externally, it's generally a good idea to use a reactor inline after or before your pump, and not to use the pump itself as the reactor. Sorry if this didn't answer your question but I thought it wouldn't hurt to share the info I've gathered.