Bam,

Is there a reason you believe you need to adjust your pH? Many
aquarists in North America would kill for your water-it is ideal for
Discus, Tetras, Apistos, and many other South American fish.

Of course if you are raising Tanganyikan cichlids, you may want some
more alkaline, harder water. In that case, CaCO3 would be the better
bet, because it raises both KH and GH simultaneously. Raising KH is
what raises pH.