Yes I know. Here is what happens think of this way. You have very little alkalinity so it takes very little acid to bring the ph down. Now as the acid breaks down since there is so little the ph rises quickly. If you add more alkalinity you can add more acid to achieve the desired ph. So as the acid breaks down there is more of it and it takes longer for the ph to rise.But potassium bircarb is alkali, I'm having problems keeping it down
But having to much alkalinity causes you to have to add to much acid and can be bad. The key is finding a balance for a stable ph.
I gave you a very conservative amount to add so if anything you may need a bit more.
Does this make sense?