This is a great article that can be found on Culture Magazine's website. It was written by DR. WHO, a Southern California expert in plant cultivation. PH and TDS balance is VITAL to hydroponic success, so pay attention:
One of the most misunderstood tools for hydroponic gardening are the pH and TDS meters. The meters come in a multitude of varieties and styles, ranging in price from about $30 for a cheap "pen" to more than $250 for a combo meter which can measure in many different scales.
pH is short for "Potential Hydrogen." This is a scale from 0-14 that measures the acidity or alkalinity of your nutrient solution. Most hydroponic nutrients perform best at 5.5-6.0 pH. If the pH is too low (acidic) or too high (alkaline), your plants won't be able to take up nutrients, especially micronutrients. This leads to nutrient deficiencies, even though the correct micronutrients are in the solution.
TDS (Total Dissolved Solids) is also synonymous with PPM (Parts Per Million). This is a measurement of the quantity of nutrients in your solution. The number can be represented in many different ways. Sometimes it is represented in EC, or Electrical Conductivity. No matter what scale you are using, it is easy to get the results that you are looking for. Normally, plants require a nutrient strength of about 1,500-2,000 TDS (or PPM). If you have an EC meter, you are shooting for about 1.8 on that scale (1 EC equals approximately 900 PPM). The more expensive meters can read EC or PPM (TDS).
When you mix your nutrients, you should start with reverse osmosis (another article in itself), which has nothing in it. It reads 0 on a TDS or EC meter. The pH would be about 7.0 (neutral). You would then add your nutrients according to the manufacturer's directions. This is a good starting point, and you should adjust the strength according to your own experience. If your TDS is too low, you should add more nutrient, mix the solution and test again until you hit the levels that you are looking for. Remember that you cannot force-feed a plant. Higher levels are not better. Too much nutrient leads to all sorts of oddities, with the results being that your plants do not grow as large or potent as they should be. Less is more.
After your levels are correct, you should use your pH meter to check the pH s. Many hydroponic nutrients are pH-buffered, which means that when adding them to the water, they automatically adjust the pH to acceptable levels, and they hold that level easier. If your pH level is at a low level, you should add pH Up (an alkaline liquid), and, conversely, pH Down if your levels are too high. Make sure that you use your pH Up and Down in very small amounts. In a 40-gallon reservoir, it might only take 10 to 30 milliliters to swing the levels .3 to .6 points either way.
You should monitor your reservoir and adjust the levels daily. Remember to change your nutrient solution about once a week, depending on the amount of nutrients that your plants are using.
The more stable that you keep your nutrient solution, the happier your plants will be, and happy plants make happy growers.