If your fish are happy and healthy, please don't mess with the pH. It is very easy to kill fish by dropping acid (pH down is HCl) in their water. pH varies with the water source. Water from wells where there are minerals in the ground (such as limestone) often have high pH and hardness. Since your water changes will be from the same water source, if you alter the pH of the tank, the water changes will change it again. Aquarium fish slowly adapt to wide variety of pH and hardness levels, but a rapidly changing pH can kill and will definitely stress your fish.
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