Help Needed! Lower leaves dying fast

[azalin]

First of all,

Hello to all and all help provided is much appreciated. I am a first time grower and in dire need of support. Here follows some information that will hoppefully help with the identification of the issue.Also the ph is going up really fast. I always lower to 5.8 but after 6-8 hours it's up to 7.5

HYDRO GROW

Strain - Pineapple chunk
# of Plants - 1
Grow Type - Hydro (DWC)
Grow Stage - Flower (5th week)
Setup - DWC
Light - 250 Watt HPS
Nutrients - General Hydronponics
Medium - Rockwool
PPM - 800 -have now lowered it to 400 in an attempt to flush
Pests - None Known

 

[azalin]

Here are some photos that i hope will help.
Also i kept the nutes at about 800ppm and have gradually lowered them to 400ppm in an attempt to flush. Temp is 75F

 

[azalin]

Some more photos