GG what’s going on here

This girl sprouted 4/18/21
In coco and perlite
Water every until run off
Using big bloom and grow big
Those bottom leaves are dying
Not using cal mag much cuz they seem better with out it
I ph water to roughly 6.0
Checked run off yesterday in was 6.3
If any one help would be much appreciated

IMO when running In coco you need to always add cal-mag. If you are using tap water have you tested the ppm?? Tap water will normally have plenty of calcium in it but probably no magnesium. To me it looks like a mag def. I need more info