CO2 idea

My plan is to have the tank water go through the wet/dry system and the CO2 will be added directly into, or just after, the power head. So the CO2 wont come in contact with any air until it is pumped back into the tank. Will that work? I will post a picture of what I am hoping my wet/dry filter will look like a little later but I am going to modify this idea (http://www.danoreef.com/DIY_5.5.htm) to make it a wet/dry filter in the first compartment, and the second will have a powerhead in it to pump water back into the tank. I want to add the CO2 into the plumbing of the powerhead pumping water back into the aquarium area. Will I still loose a lot of CO2 if I do that?
 
So here is my wet/dry aquarium plan. I am still working on the plumbing so that I can have good CO2 distribution but this is what I am thinking the tank should look like. (sorry the model is a little rough. I don't have the software to do it correctly.)

tanksetup-1.jpg


My plan is to add the CO2 into the plumbing as it heads back to the main tank area or straight into the powerhead so that, as a few of you have mentioned, the CO2 gas does not have a chance to escape unless it makes it through the plumbing into the main tank area, (but if I plumb it right, none of the gas should make it to the main tank area.)

Also, I was just curious about how well this filter will actually work. By that I mean, the canister filter I have for my 55g tank now has 3 different types of filtration but this wet/dry will only have 2, the biological (bio balls) and the mechanical (a fine sponge I will put on top of the drip tray.) My canister has some black pellet things ( I think this is charcoal?) as chemical filtration, will this need chemical filtration?

Also, what is the point of the sponge that separates the 2 sections of the filter area? I see them placed there in all sump filters I have seen so I was assuming I would need one.

Thoughts? Questions? Opinions? I would appreciate any of these.
 
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