Carbonate Hardness levels (KH)

Hi again folks.

Dupla recommends a KH level of 3.5 at least, to prevent PH fulctuations they

Since I am using a Dupla continuous PH control unit tied in with my C02, I
was wondering if a KH level of 2 will be sufficient. With a KH of 2, I do
not encounter wide PH fluctuations at all and C02 level is just sufficient
(Dupla C02 monitor gauge reads green - sufficient C02). 

I've been adding Sodium Bicarbonate B.P to my water as the tap water here in
Singapore is soft, with a hardness of zero (perfect for Discus breeders :)
). However, I find that by adding this, it actually increases the PH of my
water somewhat and the C02 keeps bubbling non-stop to bring it down. I add
5g of that for every 50 gallon change of water, which actually gives me a KH
reading of about 3-4. 
Calvin Chin