baseline is pure ro - tds 2-3 kh 0 and gh 0. Maybe is should double check kh but it is suppose to be 0. I only can measure by degree which is kind of course but will double check tomorrow.It sounds like you haven't been able to neutralize all of the carbonates. I'd recommend doing a water change and then measuring your baseline water parameters (EC/TDS and KH). If it's quite high, you may need to dilute it out first. Then, take water samples and do your pH lowering experiments outside the tank. See how that water behaves before scaling up.
For me, I had to drive the pH down to a pretty low number, at which point it took less acid to keep it there. However, you don't really want to do that with fish in the tank.