How do stop or slow down coco coir pH from rising?

Hey I wouldn't be too worried about the runoff ph. If you had water ph 5.5 in a glass and let it rest for a day you will notice an increase of ph. This is simply CO2 reacting with water and this causes the ph to raise. Since there is CO2 and Oxygen in the medium for plants to absorb, chemical reactions will increase the ph and therefore alter the ph of your runoff. I would ignore the runoff ph and not make the mistake to feed with a too low ph to adjust the runoff (stay in 5.5 - 6.3). I believe the symptoms on your girl are not being caused by incorrect ph but by the nutrient feedings, either one of these two scenarios: - PPM too high -> nutrient burn - PPM too low -> nitrogen deficiency etc. Note: Water will always dissolve first, salts (nutrients) stay in the medium for a longer time Meaning: If you've been feeding with too many nutrients (high ppm) in the past -> nutrients are building up -> high ppm in the runoff -> chance of nutrient burn To solve: just flush your girl to remove nutrient build ups and then feed with lower ppm than before and ph around 5.8-6.2 If you have been feeding too many nutrients this should work. Just measure your runoff with a ppm meter and you know for sure. !!! Important: If ppm seems to be normal -> there might be a chance that the pumice in your medium is raising the ph (pumice is definitely raising ph, don't know how much though) -> you would have to transplant your girl in a new pot with coco and perlite (perlite is ph neutral) Hope this helps Happy growing!