The best advice I can give you in my limited knowledge of water chemistry is to read the following link; whether or not the measurements are accurate remains to be seen as I am trying to create a more acidic (pH) and softer (gH) environment for my tetras and rams.
I say this because all three elements (pH,gH,kH) are intertwined and of course the nitrogen cycle can play and intereting role in altering these values.
Sorry, no simple one quick pH downer solution....you could try Aquar.Pharma. product called "pH down"....although careful because it is phosphoric acid based (many phosphates = possible algae outbreak).
http://www.fishprofiles.net/faq/begin-chem.asp
cheers,
Marc