CO2 efficiency in hard water

Hi All,
I conducted today water testing with my red-sea test kit.
The test took place before and at the early stage where the lights went on.
However I think I have asked this before, but maybe I'll be enlightened a
bit more now ;-)
It is my 100 gallon planted tank with the following parameters - week ago I
've replaced 1/3 of the water. I do replace 1/3 of all water every 2 to 3
weeks.
PH = 7.2
KH = 12
GH = 18
NH3,NO2,CL2 and FE are all = 0
The CO2 test kit presented CO2 level at over 40 PPM - At that point I've
deserted this test I trust only the PH/KH which leads to a level of 23 PPM
of CO2.
I do use a pressurized system of CO2, however I don't really see the CO2
diffuses into the water, it is a ceramic diffuser and most the CO2 bubbles
are not diffusing and are actually escaping into the atmosphere.
I will mention though that the plants and fish are happy. The water I use
are tap water.
* The tap water PH and GH are the same as in the tank. KH however is 13-14
d.
Questions:
1. Does a CO2 system is actually needed in hard water, assuming water is
replaced every 2 weeks and water parameters will not really change. PH is
rather stable.
2. Should I target to higher levels of CO2? (well ph will slightly drop and
my crypts will melt...)
3. Does any of the other parameters in hard water can effect the CO2
dissolved levels ?
4. In hard water levels I know that some plants will have trouble processing
the Macro and Micro nutritients.
5. FE levels are 0 can there be a process which the FE will bind itself to
other thing rather than to plants?
6. Is it correct to link between CO2 levels and hard water or should I look
for a new direction?

Sorry for the long mail.
Thanks,

Amit brucker