As said above, crushed coral (aragonite) or crushed oyster shell are long term solutions, they do not change pH overnight.
And just because pH is 6.8 right from the tap, doesn't mean it will stay that way 10 minutes later.
It could be the pH is low because of trapped gases (temporary) and could rise after off gassing, or...... if the water has low alkalinity (low buffering capacity) it could drop even more after being in a tank.
For my tanks, I try to determine the type species I keep, matching their needs, and my tap water.
When in Wisconsin my tap water was just the opposite of yours, pH near 8, high alkalinity and mineral content, so perfect for rift lake Africans, and Central Americans, not so good for Amazonian or Asian soft water species.
One of my holy grail fish was Uaru fernandezyepezi, but they require very soft, low pH (5-6) water parameters to remain healthy, and at $60 a pop (I usually try to acquire groups at least 6), not a realistic venture for me. I find subjecting soft water species to hard water creates chronic health problems down the road like HLLE, and digestive tract diseases.
The same may be expected subjecting hard water species to soft water. These are not always immediately apparent, but maladies like Malawi bloat come to mind.
Of course I could have invested in an RO system for Panda Uaru, but I consider those type systems wasteful (about 2/3 or more of the water that enters, becomes waste) expensive, and fiddly, even though I worked with them daily in the lab as an analyst.
Since there are over 3000 species of cichlids, evolved to living in all different water types from around the world, finding something to match my water, was the better path for me than messing with water chemistry.