Commercial pH buffers are bad for your wallet.
If you want to lower high pH water you should dilute it with RO or distilled. This will lower the total dissolved solids (TDS), carbonate buffering (KH) and general hardness (GH). The the pH drop is a side effect, basically, of decreasing the bi/carbonate level.
If you want to raise the pH you need to add carbonate buffering, baking soda is a good source. The key is carbonates and bicarbonates.
If you want to increase general hardness then there are some commercial sources like Equilibrium that are a good choice. The key is adding Calcium and Magnesium (at least if you want to adjust measurably with the average test kit).
Crushed coral, oyster shell, etc, will increase both the GH and KH of the water. It is an inexact method, but certainly works. Crushed coral is calcium carbonate, which is why it effects both.
Now, you need to ask yourself, why do you want to muck with your pH? Maintaining stable water conditions, especially in regards to the TDS, is much more important than some arbitrary pH reading.
Now that I have the preaching out of the way....
I add 1tsp each Equilibrium, baking soda, and K2SO4 (for a 75 gallon tank) each week while refilling after the 50% water change. I have extremely low, practically unmeasurable KH & GH out of the tap and my TDS reads 16... which is what some folks get out of their RO system... so that's my reason. I couldn't care less what the pH is, I just want to add the minerals and buffering.