Hey dude, sorry I hear about that.
Since you're using all mix and coco, good enough lighting, and I suppose good temps as you're not a newbie grower... Adding no nutes to the water (EC issues) ... I think it could only come down to adding too much acid to correct pH.
Please detail as best as possible your procedure, including pen brand, pH brand and quantities etc.
Your pH might be off, when did you last calibrate? Are you sure your pH buffer isn't contaminated?
Also, using acid with biobizz kills the organics, so don't be doing that. I suppose the acid can be stressing both the roots and killing goodies in the medium.
To pH correct, just use root juice, it is a quite strong 5.5 buffer. 2ml/l and you're set.
Hope it helps