Persistent low pH

runoff is never exactly what the substrate is. Also, the runoff is going to have some of the solutes from the previous fertigation added to it. Was there any visible sign of issues on the plant, or are you reacting to a measurement simply because of the measurement? flushing with 5x the volume is find for a pending catastrophe but not if everything is looking ok. That's an overreaction that only slowed your plant down more times than not, if there was not good cause to do so. Proper soilless/hydro nutes should be 100% soluble and 100% plant ready, but the competent producers also make them pH stable. e.g. my ferts come out to the right pH and don't drift easily because it's manufactured properly. Whether my tap is 8.4 or 7.0, after i add nutes it's where it needs to be. My 55-gal rez does not experience pH drift, because it's ph-buffered out of the bag. If you have a constant battle with your nutes to maintain a constant pH, then you should change brands, because that is a struggle that should not happen. Too many allower their OCD to dictate thsi stuff... "it should be this" even though it really shouldn't be that way except in your mind, then do a bunch of stressful things to make it happen. Don't do that. That called compulsory behaviour without a good reason related to the plant's health, which is what should be dictating your actions,not unresolved feelings in your head. Watering habits in soilless - coco or not, as coco is not magic, it's just another soilless substrate the same as the rest - Fertigate every time with a well-balanced 1.3-1.5 EC mix. Get 10% runoff religiously. Do not let it sit in it's piss. this runoff is integral to maintaining a consistentn equilibrium in the substrate -- this does not mean it will be exactly what you add but it will be at a consistently similar concentration after each fertigation, which is the key. Do not expect runoff to exactly match what you put in. That's called OCD and not relevant to growing a healthy plant, in fact all the extra doting will have a negative effect trying to achieve it. when you see a symptom, adjust the formula, and a new equilibrium will result very quickly. Only in extreme cases should you flood it and try to reset the substrate. A good soilless formula even out of the box should not result in such catastrophe. that would be indicative of a poorly formulated mix. N 120-130 P 40-60 K 180+ Ca 100+ Mg 75ish S 100+ These are some good spots to start for PPM. this would be a low concentration mix. Your local water will require adjustments, too. But, this would not cause any fast moving issues and you'd easily be able to observe and adjust to fit your local variables. In flower, probably wise to drop N by 20% or so, but do not cut it out completely. N is incredibly important to more than just new leaves and stem elongation (aka vegetative growth)