ZuluLaunch

I'm having an issue with my 1st grow, I'm 5 weeks in now and the run-off from 1 of my plants has spiked, pH and ppm going in is 5.8 and 1200, the run-off though is at 7.1 and 2500ppm The medium is a promix from my local store, it's 50/50 coco perlite with some extras. It's supposed to be pH neutral too. Coco Peat, Perlite, Turbo Grow, Dolomite Lime, Rock Phosphate, Powdered Kelp, Mycoroot, Diatomaceous Earth, Trichoderma & Worm Castings. I've read and read and read and even though people recommend flushing to resolve this no one can tell me what I should be flushing with, I've tried pure water, on some bad advice, and not much luck. I'm using EHG liquid nutrients to feed. at 3/4 recommended dose. I've read that if the coco is shocked with pure water(pH 5) it starts dumping salts. I'm guessing after my attempt to flush with 15l of pure water it might have hurt more than helped. Any advice welcome, I was meant to switch to flowering last week but want to get this sorted before I do

Hello fellow horticulturalists, allow me to introduce myself, my name is {redacted} from {redacted} and I got a little weary of the FB groups to get advice on growing in light of recent revelations so back to the oldskool forums. Go easy on me yeah, it's my first time