The acid sold to drop the pH in a tank will work but try to understand what you are doing with it. Fish with a reputation for liking a low pH got that reputation because it is easy to measure pH, not because the fish really care about pH. It turns out that water that is low in minerals is almost always lower in pH because most of the minerals in water comes from rocks like limestone dissolving in the water. The limestone raises the pH. Now back to your low or neutral pH fish. They are naturally well adapted to low mineral content water and you have higher mineral content than would be optimal for them. When you add the acids to the water some of the hydroxides in your water are converted to salts and the total mineral content of your water goes up because you added minerals, not down, while the pH drops. You just made things worse because you didn't understand the chemistry of your water and the true needs of your fish. If you do real research into your fish's needs, not just pH, and find that they truly require water with low mineral content, like many fish from the Amazon basin would, then treat the water by diluting with RO or rain water after you measure your starting point. WD lives in an area with very low mineral content while my tap water runs around 225 ppm of total dissolved solids and a pH of 7.8. We cannot both get away with identical water treatments to make adjustments. Each depends on the starting point and the fish's needs.