General Hydroponics pH Down Adjusting Solution - 1 Quart One 32 ounce (1 quart) bottle of General Hydroponics pH Down. Use General Hydroponics pH Down to lower the pH of your hydroponics nutrient reservoir. Add a small ammount, allow to mix, then measure the pH. Repeat until the desired pH is reached. To maximize plant growth, the pH of your nutrients should be slightly acidic. The ideal pH for most crops to fall between 5.5 and 6.5. General Hydroponics pH solutions are buffered, making it easier to control the pH of your system.