To test and adjust the pH of your nutrient solution, measure it with a meter or test kit, add pH up or pH down a little at a time, mix well, and test again. Repeat until the reading is in the range your plants need.
Step 1: Mix the solution
Add the nutrient to fresh water and mix it according to the label. Wait a few minutes for the solution to settle before testing.
Step 2: Test
Dip a calibrated pH meter or add the test drops from a kit. Note the reading.
Step 3: Adjust slowly
Add pH up or pH down in small amounts. Stir, wait, and test again. Large swings can damage roots and stress the plants.
Step 4: Check again
Test again after each top-up, because plants take up nutrients and the pH moves over time.