Is my KH to hish for plants ?

Hello All,
I have a 100 gallon planted tank.
Since I was adding to much of a CO2 with a PH of 6.2 and KH about 10
(40-45ppm)
I have decreased the CO2 amount from bubble per second to bubble per 5
seconds.
Water parameters are as follows :
PH 7.2
KH 10
GH 12
That leaves us with a CO2 at about 19ppm - Optimal! based on the Chuck CO2
table.
Ammonia, Nitrite and Chlorine are 0
The CO2 cylinder is a 500 grams so I guess it can suffice for the long run
as I do not use much of it.
I carry out water changes every 14 days about 1/4 of the tank water and the
fish load is also not to big
12 Cardinal Tetra
3 Flying fox
1 Discuss
2 Small guppies and 2 platies
20 Amano Shrimp
If I won't use the CO2, My tap water already comes with PH of 7.2 and KH of
10 - does this eliminated my need for a CO2 system ???
Do plants do better in a KH between 3-8 ?

Thanks

Amit