My tap water is very basic. But, even when I treat the water, within a few days it rises back up.
Probably has high carbonate, measured by the KH test.
Carbonate is a buffer that tends to stabilize the pH in the higher range, often in the upper 7s into the 8s.
When you add acid the pH drops temporarily, then the buffer takes control of the H+ from the acid, and the pH rises again.
To make a permanent drop in the pH do this:
Set the GH and KH where you want (perhaps about 3-5 German degrees of hardness) by mixing reverse osmosis water and tap water.
Then add some peat moss.
Start by buying a gallon of RO or distilled water from the grocery store. Mix perhaps 1 quart of it with some tap water (a cup or 2? A pint or a quart?) you will have to test and find out the right ratio) until the GH and KH read 3-5 degrees.
You will have perhaps 1-2 quarts of mixed water.
Put about 1/4 cup of peat moss in there. It is OK loose for this test. If this works for you, you will put the peat moss in a nylon stoking in the filter.
Test the GH, KH and pH daily or every few days for a week or two. For the first day the pH should drop as the peat moss works, then things should be stable, or may continue to drop slowly.
Most hatchery raised fish even if they came from soft water are usually fine in harder water unless you are breeding them.
Wild caught fish or their descendants that have only been ever raised in soft water will need soft water. Cardinal Tetras, for example still get calcium deposits in hard water.
Note I keep saying SOFT water, not low pH. These fish are looking for low GH, KH and TDS (Total Dissolved Solids), pH is much less important.