My suggestion if you don't want to waste co2 is to have an intake duct from another room that goes through the hoods and exhausts into a third room. That way you won't be pulling co2 enriched air from the grow room. The can fan can just scrub the air in the growroom, recycling it.
Personally I just pull air through an open end on the hood and remove it from the growroom, which does waste co2 but works for me...
Also, in the debate of push vs pull, I'm definately in favor of pulling air through the hoods.
Reason;
Pushing air through creates a positive pressure inside the hoods, air will find the easiest route to get out of the hood, small leaks around the glass will have air pushing out of them, making the cooling capability less efficient, because you're pushing hot air out of the cracks and into the grow room.
Pulling air through creates negative pressure inside the hoods, and light will be sucked in through those small gaps, and be removed from the room, no hot air has a chance of sneaking through a small gap.
Also, when pushing air through, if you're using flexible ductwork, it expands like a balloon, and I've had trouble keeping them attached to the hoods because of the pressure.
Lastly, always keep your ducting as simple and short as possible, excess ducts create more static resistance and your fan's CFM rating will be effected.