I actually don't want my dKh to go up along with the pH.
I just read that washing soda (NaCO3; sodium carbonate), unlike baking soda (NaHCO3; sodium bicarbonate), will only raise pH due to the lack of a hydrogen atom.
Have you considered a CO2 scrubber?
http://www.aquariumspecialty.com/catalog/product_info.php?cPath=37_799&products_id=5898
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