Carbonate hardness and Calcium

Carbonate hardness in my tank is maintained by using Sodium Bicarbonate B.P.

I usually add about 6 grams for my 75 gallon tank. Is this sufficient? My
carbonate hardness reading is 2.5 degrees German hardness. I find that
adding this stuff increases the PH of my water somewhat. I'm using C02 via
the cylinder but at 3 degrees German hardness, I have difficulty bringing
down the pH to desired 7.0 even though the c02 is bubbling day and night
from the 5kg tank into a dupla reactor. My c02 indicator in the water reads
"saturated" with a light green indicator colour. 

My question is, is a KH of 2 degrees sufficiently safe for plant growth?

Regarding calcium deficiency, I wonder if the Dupla fertilliser and
Duplaplant 24 cater to all the trace elements and stuff?

Calvin Chin