What is pH?
pH is the measure of acidity/alkalinity of a solution. 
What pH is best for growing plants hydroponically?
The ideal pH range for most hydroponic crops is between 5.5 and 6.5. 
How does pH level affect plant growth?
pH is important because it affects availability and absorption of several of the 16 atomic elements needed for plant growth. Maximum absorption of these elements is found at pH readings 5.5 to 6.5. When pH falls below this range many of the macro elements (N, P, K) have less availability, and absorption of the micro nutrients can reach toxic levels. 

How much pH Up/Down is needed per gallon?
Start out with one milliliter per gallon. Wait 15 To 30 minutes, and test your water again. Frequently you will only need 1 to 2 ml of pH Up/Down per gallon of water. You may need additional pH Up/Down if you have hard water. Flora Series is pH buffered to facilitate keeping the pH in a favorable range.