The pH levels in all hydroponics systems need to be carefully monitored and controlled. The ideal pH range in a hydroponic set-up should be between 5.6 and 6.2.
If your pH levels are outside the recommended range then your system is at a high risk of causing harm to your plants. At the very best the performance of your plants will suffer and at worst your plants will die, especially if they are left outside the optimum range for too long. pH levels can easily be controlled with the help of either a pH up or pH down solution.
An 81% solution of phosphoric acid, used to lower the pH of nutrient solutions. It is reasonably acidic of course. Though not really dangerous, pH DOWN should certainly be handled with caution. (Note - Do not store pH Up with pH Down.)