Here's what George Booth(America's own "Mr. Dupla") said-
"Since I'm always looking for ways to reduce the cost of setting up an aquarium, I was looking for a way to get CO2 into the tank without using a costly commercial reactor. Since about the only important thing is to give the CO2 enough time to diffuse into the water, I wondered if I could just bubble CO2 into the inlet of the Eheim canister filter and let it thrash around in the inlet hose, bounce around in the Ehfie-stuff, etc. And, by gum, it works!
To do it, I made a small hole in the inlet strainer and inserted one of those $0.59 right angle air-line connectors in the hole. I strapped the CO2 hose to the inlet pipe with a couple of small cable ties and connected it to the angle connector. To monitor the CO2 flow and to provide a check valve, I use a Dupla Bubble Counter, but that's not absolutely necessary - you can see the bubbles in the Ehiem hose if you watch real close.
I was concerned about bubbles collecting at the top of the Ehiem and "air locking" it, but it seems to be self clearing. About every 1/2 hour, I'll hear a gurgling and see some bubbles come out of the spray bar. I've found that the angle of the spray bar flow is important - if it's parallel to the surface, the increased surface agitation allows too much CO2 to escape to the atmosphere (I know, I know, I'm partly responsible for the Green House Effect :-). By slightly turning the spray bar towards the bottom, I still get some surface agitation and don't lose much CO2.
After setting it up, I was curious how much CO2 was being diffused into the water by my Dupla reactor and the Eheim filter "reactor".
With the Eheim, I measured the pH coming out of the spray bar using a LaMotte test kit and got 6.8 (23 mg/l dissolved CO2) with the general tank water at 7.0 (15 mg/l). This is surprisingly good for a "free" reactor."