C02 is not usually necessary below 3 wpg of lighting. 2.2 wpg is actually the low end of a medium-light tank, and most plants will be fine without it.
I've got a planted tank full of hygrophila and at 3 wpg and it does fine without C02. It doesn't grow fast and require constant pruning, but the plants are quite lush.
If you plant heavily with low and medium-light plants, you should be fine with just the light. Add C02 if you want, but it likely won't be necessary.