To maximize plant growth, the pH content of your nutrients should be slightly acidic.
Experienced growers consider the ideal pH for most crops to fall between 5.5 and 6.5.
Acid content can reach unacceptable levels for several reasons.
Among them are poor quality water and pH imbalances resulting from the use of unstable growing mediums.
Also high plant growth rates can destabilize the nutrient mix due to rapid nutrient absorption by the crop.
General Hydroponics pH Down is formulated using food-grade Phosphoric acid.
Add a little at a time if your nutrient pH is too high in order to lower the pH to the correct level.