As the plants use the nutrients, the solution weakens, but as the water evaporates from the solution, the salt concentration increases. Adjust values by either adding fertilizer or diluting with water. Use luke warm water and try to maintain a solution temperature between 60 and 80 degrees. Use a complete and soluble high quality hydroponic fertilizer according to recommendations on label. Variations either way will effect the breakdown and solubility of the nutrient compounds.
Acceptable values vary slightly with different plants, grow mediums, and hydroponic systems.
The tolerance range therefore is 6.0 to 7.0 . Use pH up and down adjusters carefully and mix in slowly and completely. Fertilizers, when added will usually lower the solutions pH value. Most of the time, as solutions are used by plants, the pH will raise, and additions of fertilizer, or a pH down adjuster will be needed. It is preferable to adjust water pH before adding fertilizer, once you are familiar with what adjustments will be required.