With uncomplicated fish, pH isn't an issue. What the value is isn't as important as the stability of the pH, and regular water changes on a weekly basis can keep the pH stable in an unplanted tank.
Then again, this works for me because my tap water is hard and comes with its own buffers. If I were to try and fiddle with the pH, I would actually be doing more damage than good. The only way I could drop the pH in my tanks would be to get an RO filter or add so much HCl (aka pH Down) as to crash the tanks.
The pH of planted aquariums is a bit different, especially if there is CO2 bubbled through the water, and those tanks might need additive buffers.
pH and buffers are completely different entities. Buffers prevent swings in pH. Usually water with a high pH (say 8-10) is also buffered because they contain alkaline salts. It is very difficult to drop the pH on buffered water, and doing so might override the buffer, thus causing a pH crash. So unless you are attempting breeding, grow-out, color enhancement, or live plants, tis best not to mess with the pH. In any of those cases, RO water would be your best bet, then you can talior your pH accordingly.
To have an exact pH of 7.0 in your tank is a fantasy. Many fish can survive a pH range, say between 6.4 to 8.2. So if your tank pH is 7.8, it is perfectly fine. The cories may not breed, but they will still be healthy and thrive.
My cories handle a small amount of aquarium salts added to their tank without a problem, and I've had them for four years now.
~~Colesea