Is this a rarity? The pH out of my tap is 7.4 so my 20 g and 40 g are at 7.4. The 40 gal is stocked similar to yours including 3 panda corys, an albino paradise fish, 10 neon tetras, a male guppy, a pleco, and a blood parrot.
Why do you want to lower the PH? There are a lot of fish that do very well in water with that PH. If you try to lower it or raise it you have to keep a very close eye on it but chances are it will do what it wants and then you have to deal with the PH yoyoing all of the time. Better to leave it where it is and keep fish that do well in water with that PH.
I found that my well water has a pH of 6.0 straight out of the tap, but when I have allowed it to become oxygenated and then take a test it will raise to between 8.0 and 8.4. This is due to the CO2 that naturally occurs in some well water. Have you always tested the water before putting it into your tank or only after the water has aged in the tank? You could have something in your tank that is naturally lowering the pH.