Not to get too confusing, but I add baking soda, keeps a real nice
buffer.... I inject CO2 which will in turn lower the pH. Starting at
7.0 without CO2, I end up dropping it to 6.4 or so, which is ok, but I
have fish that really don't care what pH it is, but adding CO2 with no
buffer, just asks for pH crashes... Keep things simple, as much as
possible... Get a gallon of tap water, declor(don't really have to)
and measure your pH and kH. Then go to Chuck Gadd, one of the generous
contributors, highly respected and one person I'd never hesitate to ask
a question, has a sight here...
http://www.csd.net/~cgadd/aqua/articles.htm
that will help emmensely with your water questions...

But ok, after you measure it, record/document pH and kH, look at adding
maybe just a 1/4 teaspoon of baking soda and watch your pH and kH take
a jump, not HUGE, but it will... do the math and see how much will work
in your tank.

It's been awhile, but that's kinda the route that helped me...
and yes, YES, always use declor, just in case...