You should be pHing the water that you are giving it, take note of that. Now take some of the runoff water and pH that. Depending on the difference of pH between water going in and water coming out you will be able to adjust the water going in accordingly. If the water is coming out too high that lower the pH some and vise versa. You can you an aquarium pH test kit from wal-mart or a pet store. They come with a test tube, test solution, and pH up and down