How to Adjust pH

The pH is adjusted by using an acid to lower it or a basei to raise it. There are many products sold by HTGsupply.com,  that you can use to adjust your pH.  I have used General Hydroponics' pH down and pH up. Many acids and  bases are extremely corrosive and dangerous, so extreme care should be used if you are not using a product labeled for hydroponic use.
I have written about adjusting the pH in soil medium on another post.
Always follow the directions of any product you use, but you should start out adding one milliliter per gallon. Wait 15 - 30 minutes, and test your water again. Frequently you will only need 1 to 2 ml of pH Up/Down per gallon of water. Keep in mind you may need additional pH Up/Down if you have hard water. The General Hydroponics Flora Series is pH buffered to facilitate keeping the pH in a favorable range.
Something I will say may surprise you, pH is not as critical as most growers believe. The main point is to avoid extremes in pH. Plants can grow on soils with a wide range of pH. However, the pH does not jump to extremes in a soil. So, the real secret is to make sure you keep your plants in a moderate pH at all times. For most plant species there is an optimum pH in the region of pH 5 to pH 6.5

The most popular componets of pH adjusters are phosphoric acid (to lower pH) and potassium hydroxide (to raise pH). Both of these chemicals are relatively safe, although they can cause chemical burns and should never come in contact with your eyes or mucus membranes. I would recommend using pH adjusters that are diluted to a level that is reasonably safe and easy to use. Concentrated adjusters can cause large pH changes and can make adjusting the pH very frustrating.

It is worth saying again, always add your nutrients to the water a few minutes before checking and adjusting the pH of your solution. The fertilizer will usually lower the pH of the water due to its chemical makeup. After adding your nutrient and mixing the solution, check the pH using whatever method you chose. If the pH needs to be adjusted, add the appropriate adjuster. Use small amounts of pH adjuster until you get familiar with the process. Recheck the pH and repeat the above steps until the pH level is where you want it to be. Once you have done this a few times, you'll know how much to give your plants the first try.

Most of the time, the pH of the nutrient solution will have a tendency to go up as the plants use the nutrients. As a result the pH needs to be checked periodically and adjusted if necessary. To start out, I suggest that you check pH minimally on a daily basis. Each system will change pH at a different rate depending on a variety of factors. The type of growing medium used, the light cycle, the kind of plants and even the age of the plants all effect the pH variations.

Good Growing
Dr. E.R. Myers