pH stability is primarily a function of your buffering capacity of your water. This is measured by your KH
(also called alkalinity). Usually, your buffer is HCO3
(bicarbonates) in most water, but if you add chemical buffers (or pH down & the like), those buffers are often phosphates. (Not good - makes algae grow).
Assuming adaquate KH
, there should be little change of your pH from original. The buffers do get used up, but you replenish it with water change.
If you have low KH
, however, you would be adviced to add buffer - best thing to use is baking soda (Sodium bicarbonate), or Calcium carbonate (crushed oyster shell, coral). This will increase your pH as well.
So, if you are happy with your current pH & stability is not a concern, then I would leave well enough alone. If you must change the pH, then filtering through peat is considered best to lower pH, and adding carbonates to increase pH.