DIY CO2 and very soft water

Greetings all. I am new to planted tanks, and had a question I was unable
to figure out the answer to using the FAQs. I am planning on adding CO2 to
my tank using the DIY sugar/yeast method. 

I know that in order to keep the CO2 from drastically lowering the pH, the
water should be relatively hard. However, since tetra's like soft water, My
tank has very soft water (<30 ppm). The current pH is 7.4, temp 77 deg F,
with amonia, chlorine / chloramine and nitrite all at undetectable levels. 
My question is, how much of a lowering can I expect once the CO2 is added? 
I do not want it to drop below 6.8, as this would be determental for the
fish. Is there any way to control this once the reaction has started?  I am
planning on turning on an air-stone or just disconnecting the CO2 at night.

TIA for your advice!


= Farzan S.                       farzan at ibm_net =