Carbonate reduction

[<IDMiamiBob@aol.com>]

I've been experimenting with carbonate reduction using muriatic acid, per Dave
Gomberg's suggestion.  I've succeeded in reducing it from 80ppm to 40 ppm, but
have now hit a "plateau" where no further effect is reached.  No mater how
much acid I add, the next day, I still get the same reading for kH, and my pH
returns to 7.6.  It appears the hydrogen chloride is gassing off without
reacting.

Any further ideas on what to try or why it is going awry?

TIA
Bob Dixon