Assuming your water is O.K. the soil needs to be amended, not the water.
When the pH of the run-off is consistently low and you are adding no nutes,
then the soil must be brought up or changed out with a transplant. With everything dialed in this should happen:Water plant with pH of 6.5. If the run-off is a lower pH the soil is holding the nutes and and the plant is drinking up just the water causing a lockout due to high salts in the soil. If the run-off is higher in pH the plant is eating up the food and needs more in the mix. If it comes out the same The feeding is just right. This is why pH is SO important in soil or hydro.