Please help!!!! leaft turns veins to brown , then the rest of the leaft!!!!!!!

hi i have 4 plants , 3 seam to be doing good. One of them the leafs started to change color to brown in the veins then passing to the rest of the leaft.
I dont find pictures of the same issue .
They are in week # 5. I did a flush a week ago. Im feeding the same to 4 plants and last feed I added cal-mag to the formula 5ml/galon.
Ph always 6.8

Pic 1 is corrupted and wont load but the top leaf in pic 2 looks like Phosphorous burn.