The pH balance of your solution is the level of acidity or alkalinity of your solution and will affect your plants ability to uptake nutrients.  

To begin, test the pH balance of your water and nutrient solution. To test your solution please follow the instructions provided with your chosen method whether it be a chemical test kit or pH pen. 

The initial reading will indicate whether it is necessary to add pH up or pH down. If your pH reading is above 6.5, add pH down. If it is below 5.5, add a little bit of pH up. Keep in mind that the pH of the solution can raise slightly after being in the water tank, so it is a good idea to aim for 5.5-6.0 to ensure you remain in range over the course of the week. 

pH up will raise the pH value of your solution making it more Alkaline. pH down will lower the pH value of your solution making it more acidic. The pH scale ranges from 1 to 14, 1 being highly acidic, 7 being neutral, and finally 14 being very alkaline. 5.5 to 6.5 pH is the ideal range, providing the optimal conditions for nutrient uptake for your plant.   

I recommend adding pH down or  slowly by adding just a few drops at a time and giving the solution time to mix before taking another pH reading and leaving it or adjusting again as needed. 

Temperature will affect your pH up and downs ability to adjust the pH value of your solution so you may need to take that into account when balancing your solution. For example the colder your solution the more you will have to add to get the same value. 

When you change the nutrient solution, keep in mind that when it is first oxygenated by the bubbler it will cause the pH to rise and will need to be checked the following morning and re-balanced to maintain that optimal range of 5.5-6.5. After this first re-balance your solution should be good for around 4 days before needing another check and potential re-balance.