RO (reverse osmosis) is the best and safest way of adjusting water parameters from hard and basic down to soft and acidic. There are other methods that work, such as peat, or rainwater, but RO is safer and more reliable long-term.
A RO unit basically removes minerals and substances from the tap water, to the point of the resulting water being very soft (basically no mineral content) and the pH usually neutral to slightly acidic. Perfect if you have--as I do--aquaria full of wild-caught fish that require this. But adding some mineral is not bad, since the presence of calcium and magnesium (mainly) increases the waters carbonate hardness and acts as a buffer to maintain a stable pH so it does not drop below where it is good for the particular fish species. It takes a bit of experimentation to get the balance between tap water and RO water where you want it, and this must not be done in the aquarium with live fish; fluctuating pH and hardness levels can seriously harm fish internally.
I cannot agree with your lfs's advice to only do this alternate weeks; it should be every week to ensure stability in the parameters. By doing even a 25% water change with tap water one week it is going to raise the hardness and pH. I responded at length on "balance" in my post in your other thread on that subject, so I won't repeat all that.
With respect to the Corydoras, most species do occur in soft acidic water; there are a few that occur in slightly basic, harder water (though not too hard) in portions of their range. To digress a moment--hardness is perhaps as much or more of an issue that pH, but the pH generally is indicative of the relative hardness--though not always--and it is useful as a guide. All else being equal, a pH of 8 probably indicates hard water, and the buffering will not result in a pH of 6 unless either the mineral is removed (via RO for instance) or acids are added to such a degree that the buffering is ineffective and a pH crash results, not good. But getting back to the corys--the common species C. aeneus and C. paleatus have a wide distribution in SA (C. aeneus even occurs naturally on Trinidad) and seem better able to adjust to harder water. Most of the rarer species are wild-caught, and such adjustments in my view are not possible long-term.