Hey just a quick question, I plan on flipping In the next couple days, only after my last transplant my ph runoff has stayed at 7 which I assume is from the cheap coco I used and in my lack of experience didn’t rinse , ppm runoff over next couple days was 1000 so I knew it was shit coco and my fault for not rinsing and buffering , so before I flower I want to flush with 5.5 ph to try lower it to avoid dramas during flower, can I flush before lights out or should I wait till the lights come back on? And if I’m in 5gal pots, would running approx 15gal through each be enough flushing or too much etc? Also any advice moving forward into the flowering stage will be appreciated 
White rhino photos in coco / 40% perlite
7 weeks old
5 gal fabric pots
Canna coco range with cultured solutions cal mag currently feeding 700ppm @ 5.8-6.1
Ppm runoff 450-550
Runoff ph 7.1 and cannot get it to drop 
You’re okay. Stop measuring runoff pH, and stay focused on input pH (fluctuating like you are) and input and runoff ppm. You’ll be okay.
I’m going to respectfully disagree with @KeystoneCops on this one. First I am new to coco also and did the same thing you did. Luckily I caught it early and was able to use my feedings to get my run off down to 6.1 before my transplant. I like to check run off to get a better idea of my soil condition and to see if they are eating. The volume of ph’d water needed will be determined by what the runoff is telling you, 15 gallons my or may not be enough.
Your intent is good, but I advise that you also trust me and ignore the pH of your runoff. Don’t even test it, because you’ll end up adjusting your input to address a non-issue, and develop lockout. But do keep an eye on runoff ppm.