The water in your aquaponics system should be between 6.8 – 7.0; however, others have experienced success slightly higher/lower as well. If the pH is too acidic, add calcium/potassium carbonate (in the form of sea shells). Add in small doses. It is much better to make small but often changes rather than a large change at once. If the pH is too basic, add a very small amount of 18% phosphoric acid. The pH will not change immediately, so wait multiple days before adding more and checking the pH. Also, high (basic) pH can lock up necessary nutrients that your plants need to survive. This can cause slow plant growth or cause plant death.