Leaves turning yellow and brown!

Hi.
All of my plants (mothers, clones) are showing almost the same sign of damage.
The plants are in different soils (coco/peat/dirt/cow manure/fishmeal, dirt/cow manure/peat, coco/peat/dirt/perlite, coco/perlite/cow manure/fishmeal/bone meal/peat, coco/perlite). I have flushed them thrice in 2 weeks, watering only with pH 6.3-6.5 water. Gave them foliar K (4 times a week since a week), some one said could be Mg deficiency, gave them pH-ed Mg water with foliar Mg spray. It seems to be advancing at rapid state now.
Please help!

For a start I would get them out of the medium you are using and re-pot with a 50/50 mix of soil and perlite and see if they recover over the course of a few days. Trim all the dead & dying leaves.... I would say you have some sort of nute lockout due to the heavy mixes you have going on. Try the KISS method first before you go for broke..

Lay off the feeds. Foliar or however. They have been loved almost to death.

I have flushed them so MUCH! How can it still be a nuke lockout.
I am so close to giving up soil and going hydro but the temp here kinda stays at 75F+ and in summers it'll be around 85F easily. Perlite is hard to comeby. Can I use rock pebbles/pea gravel instead?