If the pH is low and alkalinity is OK all you need to do is aerate, or cause bubbling turbulence and the pH will go up without affecting the alkalinity.
Here is how this works. CO2 ( carbon dioxide) in the water is in equilibrium with CO2 in the air. When the water is aerated and turbulence is created, it causes dissolved CO2 to leave the water as a gas. Because the CO2 is in equilibrium with atmospheric CO2, the CO2 that was in the water is replaced by the bicarbonates and carbonates in the water. The equilibrium reaction uses a hydrogen ion (H+) and the pH (power of hydrogen) goes up.