DIY CO2 problem

[Cheryl Trine <ctrine at andrews_edu>]

I recently (about 2 mo. ago) set up a DIY soda bottle-yeast reactor for
a 10 gal.  tank. But I sometimes have problems with wild pH swings when
I start a new bottle.  I started one yesterday and the ph went from 7.8
to 6.8 in less than 5 hours. My  fish definitely did not like it! I was
afraid I was going to lose a couple of them. After a 50 percent water
change, which raised the pH to 7.4 they perked up a bit, and didn’t just
sit at the surface. So my question is, how do you control the rate of
CO2 addition to  the tank.

My tap water parameters:
pH 8.2-8.4
<1 dGH (it us run through a whole house watersoftener, other it is 16
DGH)
17 dKH

My mix was 2 cups sugar, 1/2 t. yeast, 3/4 bottle of water, which I
swirl altogether. I use warm water to get the yeast started.

My previous 2 bottles have  maintained it at about 7.8. My initial
bottle, I had problems with the wild pH swing  as well, but I didn't
have fish in it then, so it was not really a problem.

Thanks for the help.

Cheri