The further you are into your grow the more dead dried up leaves you'll get. Take them out and don't worry unless you haven't started 12/12 yet. If your leaves are dying during veg maybe a watering issue like not enough.
fried with ferts.......P lock-out. yeah......give it a big flush. we'll see, if its end-game or not. give a good run-down of grow.
water regimen
medium
nutrition
temps
etc