" wrote in
oups.com:

A better (and much cheaper) way of decreasing your KH (carbonate
hardness, which is what's driving your pH up) is to mix your tap water
with water with lower KH, such as water from a reverse osmosis (R/O)

R/O will also address your general hardness where Acid Buffer will only
affect your pH and carbonate hardness.

Keep in mind that some of the fish the original poster is keeping are hard
water fish (guppies) and while they may not be from environments as extreme
as the rift lakes of Africa, playing with the hardness and pH may still be
completely unnecessary to begin with. Many fish are adaptable enough to
survive in the conditions his natural tap water provides and for guppies it
is pretty well right in their ideal comfort zone.

Unless the original poster is planning to go Amazonian he may in fact be
reducing the comfort level of his fish by trying to lower his pH and
hardness.