I think there's some minor technical confusion.
Some folks try to add carbonates, in hopes that it will increase their CO2 levels. After all, if you can raise your KH without raising pH, you've raised CO2 right? Wrong, this won't happen. You've added carbon in carbonate form, which is not the same as CO2, and the carbonates raise your pH anyway. Any "increased CO2" is a byproduct of non-carbonate buffers stabilizing the pH, but there's been no CO2 created, it is just showing the minor inaccuracies of using pH/KH to determine CO2 levels.
I think most folks are assuming you are asking if carbonates will add CO2, since this is a common mistake.
There are plants out there that can use carbonates, but not all do, and most do this inefficiently. It also can lead to calcium/magnesium deposits building up on the leaves if it goes on too much. (see deficiency finder's images of severe CO2 deficiencies)
Adding carbonate shouldn't be a preferred way of getting your plants carbon. However, it helps to be aware that if your plants are starving for carbon, some of them will start raiding your carbonates, as this can have adverse effects on your pH.
Last edited by mattinmd; 10-10-2014 at 02:33 PM.
Reason: added note on raising pH and pH/KH mistakes