Amit wrote:

PH 6.8~6.9
KH 9~10
GH 12~13
CO2 in ppm according to the CO2 test is 18 PPM
Ammonia, Nitrite and Chlorine are all 0 (at least these are good)

Despite the fact that the water are very hard I am kind of confused
regarding the CO2 levels.
According to some CO2 tables, the levels are HIGH 33.991 ppm (while PH 6.9,
KH 9)
Should I reduce my CO2 levels ?

Ignore your test kit. Chuck Gadd's chart or calculator is the best.

http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

With water that hard, you must be injecting CO2 to get a pH of 6.8. Am I
correct?

Also, are you using a pH controller? I ask, because you report your pH to the
tenth of a point.

Finally, why are you sad? Your CO2 is a bit high, which is easy to fix (turn
down your CO2 a tad, or if you are using a controller, set it to 7 for 27ppm at
KH9)


I know that plants can do well in hard water, but I guess I can use more RO
water ?

Why? What problem are you trying to solve?