They should be fine with the 6.5 RO; so the switch shouldn’t cause issues. Unfortunately, without the PPM measurement, we are just guessing what the problem could be. It could be too low of PPM indicating it needs a feeding, or too high of PPM causing a nutrient lockout. Without the pH measurement, we don’t know if the medium is in the acceptable range for nutrient uptake. If the pH in the medium is too high or too low, you could get nutrient lockout and see deficiencies.