The problem with using pH down type products is that the pH can fluctuate, which is not good for fish.
Let me just run through a few definitions.
pH measures the amount of hydrogen ions in the water. As acids release hydrogen ions, it is a measure of acidity. And since it is an upside down measure the more acid, the lower the pH
GH is a measure of divalent metal ions. In reality our water supplies contain mostly calcium and magnesium as divalent metal ions; others are there only in trace amounts. It is usually expressed just as calcium.
KH is a measure of the buffering capacity of water. The main water chemical that does this is carbonate, which is why buffering capacity is referred to as carbonate hardness. Water companies call it alkalinity if you come across that term on their website.
Carbonate (KH) reacts with acids and neutralises them. If the KH is high, it has to react with a lot of acid before it is used up. The products that lower pH do so by adding acids and with a high KH the pH drops for a while, then the carbonate reacts and the pH bounces back up.
The other problem is a fourth measure, TDS ie total dissolved solids. If you add something to the tank eg pH down, there are more things dissolved so the TDS goes up, the opposite of what you want.
As the others have said, the only realistic way to lower pH and hardness is by diluting your tap water with pure water - either de-ionised or reverse osmosis. But you must find the ratio of the two that gives the result you want and use that exact ratio at every water change. Using 100% tap water if you run out of DI or RO will change the hardness and pH and harm the fish. Now is the time to experiment to find the ratio and replace all the current water with the mixed water, before you get fish.
The other alternative is to stick to fish that like hard alkaline water.