What sort of buffers do you add? I have had problems keeping the water
stable with liquid buffers and switched to a small amount of crushed
coral to help buffer. When I had a stable PH and KH that I liked, I
started with CO2. I also found that the crushed coral approach worked
much better for the KH than buffering solutions.

If you are using liquid buffers and want to try corals/seashells my
recommendation is to add slowly and in a manner that you can remove
some/all if you add too much. For me, it only took a few tablespoons.
(it is hard to buy a few tablespoons of crushed coral, so now I might
as well do a marine tank with the extra right?)

There are some informative articles on how much CO2 will lower the PH.
Hopefully some other people can give you better info.