Because hydroponics gardening relies so heavily on nutrient-laced water (nutrient solution), the water must be as ideally suited to the plant as possible. That includes pH level. Pure water has a pH of 7, but most plants want conditions more acidic than that. Tomatoes, for instance, like a pH of 5.5 to 6.5. You must match the pH of your solution to a plant's needs when you mix up your nutrient solution and then keep checking the pH level, especially if your hydroponic system recycles water, which changes the pH of the nutrient solution as you reuse it.
Research the pH needs of your plant.
Mix the hydroponic fertilizer with the water according to the fertilizer's package directions.
Check the pH level with the testing strips or by using the kit.
Adjust the pH by strictly following the instructions on the pH adjuster. If not using adjuster, add small amounts of baking soda to a gallon of solution if the solution is too acidic or a few drops of white vinegar if the solution is too alkaline.
Recheck the pH of the solution.
Adjust the pH until it matches the needs of your plant.
Check the pH of the solution daily until you get a sense of the pH fluctuations of your particular system, then as needed after that, adjusting the pH accordingly.