The roots need oxygen, and you can easily aerate the water a couple ways. That will raise the dissolved oxygen levels in the water. But as far as Co2, that's another story. I know there are products that manufactures claim by adding to the water will increase the Co2 absorbed by the plant. But as far as I have seen there is nothing by any real research to substantiate that claim. Though I do believe the roots can absorb some Co2, but not in any significant amounts. it's not the job of the roots to uptake Co2. And even if you add Co2 to the water, 99% or more of the Co2 will be absorbed through the plants leaves.

If you are growing in a well ventilated greenhouse, you wont need to worry about the plants getting enough Co2. The plants willstill deplete the Co2 from the air. But by ventilating the greenhouse with fresh air, you will be continually replacing the Co2 in the air the plants have useed. Thus, you wont actually be depleting the co2 at all, as long as it's well ventilated.

Also, I just thought I would mention that just by increasing Co2 levels, that wont necessarily make the plants absorb it. The plants Co2 consumption is also related to environmental conditions like light levels, humidity, temperature etc..

I grow chiles in an semi-enclosed environment and have experimented with increasing Co2 and the chile plants don't notice. If the environment is not sealed, the Co2 diffuses too fast to make a difference.

Thanks very much for this info. I didn't think it was necessary, and I knew that they absorb the majority through the leaves, I was just wondering as I had heard some of those manufacturers claims. Thanks homehydro for verifying that for me. The greenhouse is very well ventilated. Willard3, I wanted to put the CO2 into the water as to reduce diffusion into the environment.

For those of you that are using CO2 injection, I am curious what you are using for emitters/difusers? Do you just use the garden variety available for aquariums?

No co2 in your water will kill your root system. Also co2 is a heavy gas and will sink to the floor where it won't help the plants. Easiest thing to do is use baking soda and vinger. This stuff will foam once you mix them so be carefull. Use two fans, one low and another high at opposite ends of your space. If the rom isn't to large sit in there awhile and enjoy your self. You give off plenty of co2 yourself.