Here is a hydroponic pH chart, this varies dependent upon your individual system setup and environment. Nonetheless, this is a valuable reference tool, and stresses the importance of dialing in your pH accurately. Every time you think you have a problem with your grow, check your pH first. 

This chart illustrates the nutrient uptake levels for each nutrient at specific pH readings. 

Just a couple of very basic points on hydroponic pH maintenance:

-a pH of 7 is totally neutral. It is neither basic nor acidic. Distilled water has a pH of 7.

-The lower the pH number, the more acidic the solution is, the higher the pH number, the more basic.

-Make sure you adjust your pH after you add your nutrient solution(s), and then check back 12 to 24 hours afterward to make sure the pH levels have stabilized.

-Get a pH meter, this eliminates the guess work of color based solutions.

To help you keep your pH on point, we have digital pH meters from Hanna for $70.00, and General Hydroponics 

pH Up or Down (Quart) for $10.00 each.