Using chemical to lower pH almost never works. What happens is that the water's buffering capacity, expressed as KH (which stands for carbonate hardness) must be exhausted more or less before pH can be lowered. Often, this means the carbonates are bound by phosphates (as in Proper pH) or neutralized through the use of acids (e.g., pH Down).
The problems with these approaches are many. Adding phosphates increaseses the TDS (as already mentioned) and actually makes the water less hospitable for soft water fish. Also, the phosphates make great food for algae and massive green water outbreaks are common.
Using acids may work, but by the time the carbonates are burned up in this method, the water has very little buffering capacity to resist further changes in pH. (Before the buffer is completely burned up, you see an immediate decrease in pH, while the acid additive is free, then a ph bounce as the acid interacts with the carbonates and is used up. If you treat with acid long enough, the bounce is eventually eliminated, because the buffer is gone.) A little more acid can cause pH to plummet, a bit more carbonate (like when you add fresh water or do a water change) replenishes the buffer and pH will shoot up. Both of these are bad for the fish.
Using reverse osmosis water is probably the best solution, although the upfront investment can be a bit pricey.
HTH,
Jim